The effect of insulin and oxytocin on glucose oxidation and lipid synthesis has been studied in isolated fat cells. Insulin and oxytocin both stimulate glucose utilization in fat cells; however, these hormones act through different mechanisms as shown by the following findings: 1) Doses of both hormones, which individually give maximal responses, when combined produce additive effects; 2) N-carbamyl-O-methyloxytocin, a specific oxytocin antagonist, markedly inhibited the lipogenic response of fat cells to oxytocin without affecting * the insulin response; 3) omission of calcium from the external medium completely abolished the metabolic effects of oxytocin, but did not influence the stimulatory effect of insulin on glucose utilization in fat cells. (Endocrinology85: 1092, 1969)