These studies were designed to examine whether interrelationships exist between serotonin and prostaglandin E (PGE) during regulation of insulin secretion in dogs in vivo. In our studies serotonin was found to inhibit insulin responses to intravenous glucose. This inhibition was not reversed by complete adrenergic blockade provided through combined phentolamine and propranolol pretreatment. This property of serotonin is similar to that of PGE which also inhibits glucose-induced insulin secretion in vivo independently of adrenergic activity. To investigate whether these effects of serotonin and PGE are related, studies with methysergide (a serotonin antagonist) and indomethacin (a PGE synthesis inhibitor) were performed. Methysergide reversed the effects of both PGE and serotonin. In contrast, indomethacin did not diminish the inhibitory effect of serotonin upon insulin secretion. It is hypothesized that endogenous serotonin may play a role in the inhibitory effect of PGE upon insulin secretion in dogs in vivo.