The effect of thiourea and its derivatives, including methyl- and propylthiouracil as well as the imidazole derivative thiamazole on glucose-induced insulin secretion from incubated rat pancreatic islets was studied. Additionally, the effect of a single oral dose of propylthiouracil on plasma insulin and glucose tolerance was tested in anaesthetized rats. In the presence of 2.8 mM glucose, neither thiourea nor methylthiouracil, propylthiouracil or thiamazole stimulated the secretion of insulin from pancreatic islets. However, in the presence of 11.1 mM glucose all of the above compounds augmented the insulin-releasing properties of glucose in a concentration-related manner-propylthiouracil being the most potent drug. Propylthiouracil (100 and 200 mg/kg body weight) significantly augmented insulin secretion in vivo in response to i.v. glucose (0.5 g/kg). Accordingly, the rate constant of glucose elimination (K-value) was increased. The data suggest that thiourea-containing chemical compounds sensitize pancreatic islets to the insulin-triggering action of glucose.