TORONTO - The Marlboro Man may look good in the saddle but he is likely not so good in bed, new research shows.

Non-smoking men have more sex and enjoy it more than smokers, say scientists from the American Institute of Andrology in Lexington, Ky.

But lead researcher Panayiotis Zavos said in a presentation yesterday that scientists do not fully understand why.

The researchers studied 290 men between the ages of 24 and 36, and found that the non-smokers had sex nearly 12 times a week on average, compared with six times a week for the smokers.

The non-smokers also rated their enjoyment at nearly nine on a scale of one to 10, compared with five for the smokers.

The research was presented yesterday at a conference of the Canadian Fertility and Andrology Society and the American Society for Reproductive Medicine in Toronto.

The research is surprising, said fertility researcher Susan Benoff of the New York University School of Medicine, but fits with several recent studies showing that smoking alters bevaviour. "I've seen three or four reports in the past few weeks about smoking causing alterations in behavior," Dr. Benoff said.

"Would I have expected this?" Dr. Benoff said. "No, but I'm not as surprised as I would have been."

The smokers in the study smoked more than 30 cigarettes a day and had done so for more than seven years.

The non-smokers had never smoked and the wives and partners were all non-smokers.

It has long been known that smokers are less fertile than non-smokers, but the reasons are not completely clear.

It may be, the researchers say, that chemicals in cigarettes impair the production of sperm, making male smokers less likely to start a pregnancy.

In this study, the researchers found that the non-smokers on average spend 2.6 years trying to conceive, compared with more than three years for the smokers.

It's even less clear how cigarette smoking affects sexual behaviour and enjoyment, he said. "It is possible that smoking may act at different levels in the body, diminishing ... sexual frequency and satisfaction," the researchers concluded.

But the study also concluded the data should be "a warning for anyone wishing to reproduce and particularly for those having difficulty in conceiving."