''Sexual activity seems to have a protective effect on men's health,'' Dr. George Davey-Smith's team concluded after analyzing death rates of nearly 1,000 men from 45 to 59.

The researchers said they undertook the study because few findings existed on any link between patterns of sexual behavior and death, and they challenged epidemiologists elsewhere to conduct similar studies, particularly among women, for whom very little relevant data exist.

Dr. Davey-Smith's team assessed the existence of heart disease in the men when they entered the study from 1979 to 1983. After explaining the purpose of their question, they asked the men about the frequency of sexual activity. The answers were put into categories ranging from ''never'' to ''daily.''

The question about male sexual activity was dropped midway through the enrollment period because local doctors suggested that many men did not like talking about sex, said Dr. Davey-Smith's team from the University of Bristol and Queen's University of Belfast.

The participants' names were flagged in the British national health service's central registry and the researchers were automatically notified if they died. The death rate was analyzed 10 years after the participants entered the study.

Men who said they had sex twice a week had a risk of dying half that of the less passionate participants who said they had sex once a month, Dr. Davey-Smith's team said.