This nation has just witnessed a week-long, largely grass-roots effort to awaken the American people to the facts of nuclear life. Nuclear freeze resolutions have been passed in hundreds of town meetings. Out in California a freeze initiative has received 700,000 signatures, more than twice the number required to get it on the November ballot. Between the Ground Zero movement and the nuclear-freeze movement the American people have been exposed to massive doses of nuclear information. Will it do any good? Can this kind of citizen effort move governments?

It is a measure of the age in which we live that trendy topics appear on the screen for a few weeks and then fade away, a week or two after making the covers of Time or Newsweek. We are a nation easily infatuated with worries. We thrive on fads. Few things, with the possible exception of the physical fitness movement, have taken hold in the national psyche and changed the way we live. Open marriage has gone the way of Perrier water: People may still be sampling it, but no one is talking about it anymore.

We embrace causes with all the constancy of teen-agers. But there are exceptions, and while no one can predict the outcome of the new nuclear awareness, we can look at two equally grass-roots movements that have brought permanent change to our way of thinking and doing business.

Candy Lightner was working as a real estate agent in a suburb of Sacramento when her 13-year-old daughter was killed by a drunken driver on May 3, 1980. "Four days later I found out that he probably wouldn't even go to jail" because of the lenient approach to drunk drivers. "You're talking about someone who killed my daughter and left her on the road to die."

She founded Mothers Against Drunk Drivers, a grass-roots organization that in less than two years helped produce a dramatic change in public attitude toward drunken driving, won passage of tougher laws in nine states including Maryland and Virginia, got task forces appointed in others, flooded Congress with letters urging support for a presidential commission on drunken driving, which was recently appointed by President Reagan. "California now has the toughest drunk driving laws of any state," says Lightner. "That was due solely to us.

"I didn't know anything about any of this," she says. "I was was not a registered voter, I had never read a law in my life, never spoken in public, never formed a corporation. I'd never been to court, never spoken to a judge. Everywhere I went people said, 'You won't get anywhere; it'll take years.' I was so bitter, I just kept plugging along."

Doug Johnson was with the Third World Institute, a Catholic foundation at the University of Minnesota, when he was approached by representatives of the Interfaith Center on Corporate Responsibility. The year was 1976 and missionaries and pediatricians in Third World countries had for some years been drawing attention to the connection between malnourished, dehydrated babies and their mothers' use of improperly diluted infant formula. Churches in the United States had been successfully pressuring American formula manufacturers to reform their marketing practices, but they had no leverage against Nestle, the multinational Swiss firm that had half of the market.

Four small organizations in Minnesota started INFACT, the Infant Formula Action Coalition, with Johnson as chairman, and on July 4, 1977, launched a state boycott of Nestle products. The boycott went national that November and a year later spread to Australia, New Zealand and Japan. England and Sweden joined in 1980 and West Germany joined in 1981.

The result of this international movement was the adoption of a World Health Organization code restricting the marketing procedures of infant-formula manufacturers to protect consumers throughout the world. The only nation to vote against the code was the United States, in what became one of the first major embarrassments of the Reagan administration. Now 95 groups in 65 countries monitor enforcement of the code.

MADD and INFACT were not tackling the fate of the world and hatred among nations. But they were tackling life-and-death issues and going up against ignorance, apathy, America's love-hate relationship with alcohol and a powerful multinational corporation. The success of their efforts bodes well for the grass-roots movement to make the nuclear weapons situation more rational. It may not seem like it, at times, but individuals can still make a difference.