The effect of additions to the culture medium of some natural or synthetic corticosteroid hormones was studied in cultured rat islets of Langerhans. The steroids decreased glucose-induced insulin release. The extent of inhibition by dexamethasone was 18-55%, prednisolone 23%, hydrocortisone 21% and aldosterone 18%. None of them affected the basal secretion of insulin or had any effect on diameter or insulin content of the islet. The inhibitory action of dexamethasone on insulin release was observed in the range 63 nmol/l to 6.3 mumol/l. At 6.3 mumol/l during two h, dexamethasone (a) inhibited insulin response to glucose concentrations above 5 mmol/l (b) caused a delay in the first phase and markedly reduced the second phase of insulin release of perifused islets, and (c) decreased the incorporation of [H3]-leucine into total islet proteins without affecting [H3]-leucine-incorporation into insulin plus proinsulin. It is suggested that steroids, by directly acting on the islets of Langerhans, may modulate the insulin-release response to secretagogues.