The concentration of serotonin in the serum, duodenum and total pancreas and the specific fluorescence of serotonin in the islets of Langerhans, as well as the levels of glucose and insulin in the serum, were assayed hourly in normal rats following the administration of L-tryptophan. This treatment produced a significant increase in the concentration of serotonin in the serum and tissues, beginning at 60 min and lasting a total of 240 min. During this time, the serum glucose levels showed the reverse curve. No fluorescence was detected in the islets of Langerhans. When the animals were pretreated with an aromatic L-aminoacid decarboxylase inhibitor, no significant changes were observed in either the serum serotonin or the insulin levels; however, the decrease in the serum glucose was smaller but still present. The results obtained with the rat-pancreas-slices incubation showed a stimulatory effect of serotonin upon insulin secretion. These results could suggest that serotonin might act as a stimulatory agent on insulin secretion and also contribute through this effect to the tryptophan-induced hypoglycemia. 1 Presented in part at the Seventh Congress of the International Diabetes Federation, Buenos Aires, 23-28 August 1970, and published in abstract form in Excerpta Medica 209:48(1970).