Lay men and women have moved into key leadership positions in Roman Catholic parishes across the country, supplementing and, in many functions, replacing priests over the past couple of decades, a study released yesterday by the University of Notre Dame discloses.

The demand of the Second Vatican Council 20 years ago for all Catholics -- priests and laity -- to share the responsibility for the life of their parish has taken root, changing sharply the complexion of local churches in this country, the study found.

"The picture of a parish where Fr. O'Brien took care of God, Sr. Cerita ran the school and the people met their mass obligations and said Hail Marys would be a woefully inadequate stereotype of U.S. Catholic parishes in the 1980s, if ever," it said.

The increased involvement of lay Catholics reflected in the study is a ray of hope for a church confronted by a drastic decline in the numbers of young men preparing for the priesthood and, consequently, by a projected shortage of priests available for service by the year 2000. Of the 57,891 priests in the church today, many are approaching retirement. Serra International, a group committed to recruiting men into the priesthood, says seminary enrollment has dropped from 39,838 in l967 to 11,262 today.

The report -- "The Notre Dame Study of Catholic Parish Life" -- was begun in 1981 and is based on an analysis of 1,100 Catholic parishes across the country. It was released by Msgr. Joseph Gremillion, director of Notre Dame's Institute for Pastoral and Social Ministry, and David C. Leege, director of the Center for the Study of Contemporary Society.

The stereotypical pre-Vatican II parish in which the pastor alone leads the flock has ceased to exist, the study said. It found that "beyond the parish pastor, 83 percent of the leadership within Catholic parishes, paid or unpaid, are laypersons. Even among the paid staff with responsibilities for key programs, 57 percent are lay."

Lay men and women, paid or unpaid, are involved in everything from finance and administration to liturgy, religious education and a variety of ministries. The ministry is shared by priest and laity in 36 percent of the parishes studied while 64 percent involved a combination of priests, laity and nuns in the ministry.

The study also found that nearly half the rank-and-file church members take part in parish activities. Spiritual renewal activities attract the largest number -- 24 percent. Eighteen percent take part in charitable or social programs of the church, and 10 percent regularly take one of the liturgical roles open to lay persons. Five percent serve on parish councils or an equivalent form of parish decision-making.

The study found that 87 percent of the parishes studied have, will have or have had parish councils, in response to the Second Vatican Council's mandate for shared responsibility in directing church affairs.

Such units have raised problems, however. The study found that "while shared responsibility symbolized in parish councils is important, effective decision-making is either more centralized -- in the pastor -- or decentralized to the level of those lay persons and staff especially interested in a particular type of ministry or program."

The study also found that fewer than 15 percent of Catholics regularly attend mass outside the parish in which they live.