AUSTIN, Texas --- Since Viagra was introduced 10 years ago, the male impotence-curing drug has become a multibillion-dollar industry -- and the butt of late-night jokes.

But this is no joke.

Scientists at Texas A&M University say watermelon, the juicy fruit that is popular during summer picnics, has ingredients that deliver Viagra-like effects to the body's blood vessels and might even increase libido.

"Watermelon is not just a delicious fruit. It is healthy, and it may even improve a man's sex life, just like Viagra," said Dr. Bhimu Patil, the director of Texas A&M's Fruit and Vegetable Improvement Center in College Station. "We always knew it was a healthy fruit, but the more we study, the more amazed we are."

Healthful ingredients in watermelon and other fruits and vegetables are called phytonutrients, which help the human body trigger healthy reactions and relax blood vessels, pretty much the way Viagra does, Dr. Patil said. Moreover, scientists know that when humans consume watermelon, the ingredients work wonders on the heart and circulation system and help maintain a good immune system.

In addition, although watermelon is 92 percent water, the remaining 8 percent is loaded with lycopene, an antioxidant that protects a person's heart, prostate and skin health, Dr. Patil said.

Best of all, a slice of watermelon is much cheaper than a Viagra pill, and there is no fear of a side effect that some drugs have on humans.

Penelope Perkins-Veazie, a researcher at the U.S. Department of Agriculture's South Central Agricultural Research Laboratory in Lane, Okla., reported in a 2002 study that nutritionists are pleasantly surprised about what they are learning about watermelons.

"We think there are a lot of potential uses for watermelon that are just beginning to be explored," Ms. Perkins-Veazie said at the time. "It can be a so-called functional food -- one that can help prevent certain diseases."

More recently, nutrition experts at the University of Texas Southwestern Medical Center at Dallas reached a similar conclusion. They reported watermelon also is rich in vitamins A and C, has no fat and is low in calories.

"Watermelon is a great fruit to help you hydrate and cool down in the summer heat," Lona Sandon, an assistant professor of clinical nutrition at UT Southwestern and the spokeswoman for the American Dietetic Association, said in the June 14 report. "Also, since it is made up of mostly water, purÃed watermelon makes a refreshing drink."

Dr. Patil said scientists would like to see Americans eat more fruits and vegetables. Ideally, people should eat five fruits and vegetables a day, but on average they eat 2.4 -- which helps explain why many Americans are overweight and might eventually develop diseases such as type-2 diabetes.