Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Jeffrey M. Friedman
When Friedman started his own laboratory at The Rockefeller University, he turned his attention to the question of weight regulation. Working with a special strain of mice, he set out to identify the hormone that normal animals use to control their appetite - a molecule that was missing in the plump rodents. After eight years—on May 8, 1994, at 5:30 a.m.—he found what he was looking for: evidence that he'd located the gene that produces the hormone he later dubbed "leptin", after the Greek word for "thin" (λεπτός "leptos"). "It was astonishingly beautiful", he says of the x-ray film that nailed the gene, a piece of data that now hangs on his office wall.