The Vatican is quietly conducting two sweeping investigations of American nuns, a development that has startled and dismayed nuns who fear they are the targets of a doctrinal inquisition.

Nuns were the often-unsung workers who helped build the Roman Catholic Church in this country, planting schools and hospitals and keeping parishes humming. But for the last three decades, their numbers have been declining - to 60,000 today from 180,000 in 1965.

While some nuns say they are grateful that the Vatican is finally paying attention to their dwindling communities, many fear that the real motivation is to reel in American nuns who have reinterpreted their calling for the modern world.

In the last four decades since the reforms of the Second Vatican Council, many American nuns stopped wearing religious habits, left convents to live independently and went into new lines of work: academia and other professions, social and political advocacy and grass-roots organizations that serve the poor or promote spirituality. A few nuns have also been active in organizations that advocate changes in the church like ordaining women and married men as priests.

Some sisters surmise that the Vatican and even some American bishops are trying to shift them back into living in convents, wearing habits or at least identifiable religious garb, ordering their schedules around daily prayers and working primarily in Roman Catholic institutions, like schools and hospitals.

"They think of us as an ecclesiastical work force," said Sister Sandra M. Schneiders, professor emerita of New Testament and spirituality at the Jesuit School of Theology at Berkeley, in California. "Whereas we are religious, we're living the life of total dedication to Christ, and out of that flows a profound concern for the good of all humanity. So our vision of our lives, and their vision of us as a work force, are just not on the same planet."