New York - For Alison Schneider of Manhattan's posh Upper East Side, introducing women to a 3,300-year old observance means nothing short of "preserving the Jewish community" and "bringing blessing into their lives."

That observance is mikvah, the spiritually purifying immersion in ritually pure water required each month of married menstruating Jewish women in order to keep the biblical laws of family purity (taharat hamishpacha). Schneider is one of dozens of people involved in helping to build a new mikvah on Manhattan's Upper East Side to enable hundreds of families to pursue the practice.

For more than three millennia Jews have considered the mikvah to be the very foundation of Jewish family life and have gone to great lengths, even in the worst of history's circumstances, to build mikvahs and to observe the immersion ritual.

Women braved walks alone at night through forests and desolation, cracked through frozen rivers to immerse themselves in icy waters and risked imprisonment and other cruel punishment to fulfill the commandments of family purity. The remnants two mikvahs built by the Jews in the Masada fortress while under Roman siege can still be seen there today.

World upheavals, assimilation, misinformation and generational gaps all contributed to decreased observance of this critical Jewish practice in the beginning of the second half of this century. The Lubavitcher Rebbe, Rabbi Menachem M. Schneerson, of righteous memory, placed much emphasis on spreading the knowledge and practice of Family Purity and succeeded in redirecting the trend toward increased observance.

Alison Schneider, 26, says that her mother, philanthropist Dr. Patricia Cayne, and herself joined in working on the new Upper East Side project because, "Many people my age are seeking to add meaning and spirituality to their lives, especially to their marriages and to having children. When I explain to them that mikvah does this, they are eager to adopt the observance of mikvah."

Schneider takes two to three new brides, or women trying to have children, to the mikvah each month, explaining to them both the laws and some of the spiritual associations of the practice. "It's a powerful experience. Every person I've gone with feels spiritually rejuvenated after immersing. Having a mikvah in the neighborhood will make many people who live nearby more willing and able to use it," says Schneider.

Says Cayne, "This is something the women of my generation are doing for our daughters, who are bringing religion and spirituality into their marriages and families."

The Upper East Side mikvah project was initiated by Rabbi Ben Tzion Krasnianski and his wife Chani, Chabad-Lubavitch emissaries to the neighborhood. Shortly after coming to the Upper East Side seven years ago the Krasnianskis recognized the need for building a mikvah that would enable Jewish women to pursue the commandment of ritual immersion without having to travel to other neighborhoods. It turned out that this was something that the Lubavitcher Rebbe had pushed for for years in private conversations he had with people from the area.

Manhattan's large Jewish community has long been served by one mikvah in the Lower East Side and a second on the Upper West Side, a lopsidedly low ratio, especially compared to that of many smaller Jewish communities around the globe. The recent growth in both Jewish population and Jewish observance on the Upper East Side, home of one of the world's wealthiest and most influential Jewish communities, contributed to the demand for a new facility to serve this population.

The Jacques and Hannah Schwalbe Mikvah, at 419 E. 77th St., is expected to be completed by the fall of 2000. Its elegant design, by internationally-acclaimed architect Michael Khidekel, will feature three immersion pools, including one equipped for use by people with disabilities, 12 preparation rooms and a bridal suite.

The converted brownstone will also host The Schneerson Center for Jewish Life, which will include a nursery school, a youth center, an adult and a fifty-plus education center, and a holiday and Sabbath community celebration center, in an elegant setting appointed with sweeping archways and a rooftop garden.

In all $4 million are needed for the center, with $2 million raised so far.

"For a long time the Upper East Side was considered to be home to some of the most assimilated Jews in America," Rabbi Krasnianski says. "By the time they became American success stories, the Jewish experience had been left behind. But that is changing. The demand for Jewish expression here is exploding."

Krasnianski notes that all of the area synagogues, as well as the Chabad-sponsored services he conducts in the neighborhood on Friday nights, are filled.

As if to prove him correct, hundreds turned out to a recent event in New York's venerable Guggenheim Museum for the mikvah and the building project. Among them was Rabbi Haskel Lookstein, rabbi of Congregation Kehilath Jeshurun on East 85th Street and a third-generation rabbi on the Upper East Side.

Rabbi Lookstein points out that mikvah observance has increased over the last years and there is also more "openness" about the topic. "Forty years ago I would hesitate before discussing these laws in a pre-marital conference," Lookstein says. "Today I teach a course in mikvah at Ramaz High School."

"For the longest time there has been only one mikvah in upper Manhattan, on the West Side," says Rabbi Lookstein, "and it has been basically inaccessible to women of this area on the Sabbath and holidays. Having a mikvah here is a huge boon to observance of this commandment."

Rabbi Lookstein believes that the mikvah's stylish execution will enhance its use. "The fact that this mikvah will be aesthetically appealing and very inviting will make a very big difference," says the rabbi. "This is all part of Rabbi Krasnianski's vision. It's all his work, and he is undertaking it in the most modest way, fitting to the nature of mikvah."

Peter Schwalbe, who is endowing the mikvah in memory of his parents, echoes Rabbi Lookstein's enthusiasm. "Mikvah is incredibly important. My parents were involved in community service all their lives. I truly believe that helping build this mikvah for the entire community is a wonderful way to perpetuate their memory. People will use this, because it is here."