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.
Miguel Ondetti
Miguel Ondetti moved to the gastrointestinal hormone group in the mid 1960s. The first hormone to be studied by Ondetti's group was cholecystokinin, a digestion hormone. This product presented unique challenges because of the small amounts used in reactions. Unfortunately, bioassays were not quantitative enough to use for results and chemical reactions did not run well as such low amounts.