Precancerous changes are showing up far less often in the lungs of men who smoke today's lower-tar, lower-nicotine cigarettes than they did in men who smoked the stronger cigarettes of the 1950s.

This finding -- made by examining the lungs of men of both eras -- is reported in the lead article in today's New England Journal of Medicine, the country's preeminent medical journal.

Taken together with indications that lung cancer deaths may already have started to taper off in younger men who have smoked milder cigarettes, there should eventually be a significant decline in lung cancer, say the study's authors.

The authors are Dr. Oscar Auerbach of the East Orange, N.J., Veterans Hospital, one of the world's leading authorities on lung cancer's effects, and the American Cancer Society's top statisticians, Dr. E. Cuyler Hammond and Lawrence Garfinkel.

Aside from theory and statistics, the new finding is the first solid indication that there may indeed be a good effect of what a National Cancer Institute scientist last year called today's "less hazardous cigarettes."

The Cancer Institute scientist, Dr. Gio Gori, also emphasized that the only truly "safe" cigarette is the cigarette that is not lit. He was nonetheless severely criticized by his institute and Department of Health, Education and Welfare superiors, who have been crusading against all cigarette smoking.

Dr. Lasalle D. Leffall, Howard University surgeon and current American Cancer Society president, cautioned yesterday that "the nation's guard against cigarette health hazards must not be allowed to relax," and "there is no safe cigarette."

He pointed out that the new research did not take into account the deaths among smokers caused by heart disease, other lung diseases and other forms of cancer.

He enthusiastically called the new report "optimistic," however, and said it indeed suggests that smokers can reduce their lung cancer risk by switching to low tar/nicotine brands if they can't quit entirely.

Dr. Auerbach is the pathologist who first showed that ominous changes in the inner surfaces of the bronchi -- the lung's many airways -- are more common in cigarette smokers than nonsmokers. He found the changes increase when smoking increases or when it continues for years.

In his new study, he examined more than 20,000 sections taken at autopsy from men who died of caused completely unrelated to lung cancer. He studied lung samples from 211 men, 154 of them regular smokers, who died in 1955-60, and samples from 234 men who died in 1970-77, 181 of them smokers.

In men who smoked one to 12 cigarettes daily, these changes occurred in 2.6 percent of the 1955-60 group but only.1 percent of the 1970-77 group.

In men who smoked 20 to 39 cigarettes a day, they showed up in 13.2 percent of the 195-60s but only.8 percent of the 1970-77s. In men who smoked 40 or more cigarettes daily, they appeared in 22.5 percent of the 1955-60s and 2.2 percent of the 1970-77s.

Auerbach had no data on the precise brands of cigarettes each man had smoked. But, with his co-authors, he pointed out that since the 1950s, filters and changes in tobaccos have resulted in "a large and continuing drop in the tar and nicotine content of the mainstream smoke" of American cigarettes.

"Thus," he concluded, "everyone who has been a habitual cigarette smoker for 25 years or longer must be smoking cigarettes with less tar and nicotine," and, also, many smokers have deliberately chosen low-tar and low-nicotine brands.

In an accompaying editorial, Dr. Alfred Fishman of the University of Pennsylvania said the new findings are consistent with some mortality trends. But he warned that "it is not yet possible ot prophesy with any confidence" that lung cancer rates will indeed change, and "we shall have to wait and see."