Interleukin 1β has been proposed to play an essential role in the pathogenesis of IDDM by direct interaction with the pancreatic beta-cell. Glucocorticoids are widely used as immunosuppressive agents and have been suggested to interfere both with the production and action of interleukin 1. The aim of the present study was to evaluate the interaction between cortisol and interleukin-1 on the pancreatic beta-cell function in vitro. Newborn rat islets were precultured for seven days before they were exposed to interleukin 1 with or without addition of cortisol. The release of insulin to the culture medium was followed and the insulin content and biosynthesis were measured after one week in culture. Cortisol, 10−6 mol/l, resulted in a 50% lower rate of release during the culture period and a similar reduction in storage and biosynthesis of insulin in the islets at the end of the culture period. In the control islets interleukin 1, 0.5 μg/l, resulted in an early increase in insulin release followed by a marked reduction. In the cortisol-treated islets interleukin 1 increased the release up to 72 h followed by a moderate decrease. In the control islets, interleukin 1 reduced the insulin content to about 50%, whereas in the cortisol-treated islets interleukin 1 resulted in an even greater reduction, to about 30%. This additional effect of cortisol seems not to be due to an augmented cytotoxic effect of interleukin 1 as indicated by the DNA content of the islets and the viability of the cells. In conclusion, cortisol treatment augmented both the early stimulatory and the late inhibitory effects of interleukin 1 on insulin release and biosynthesis in isolated pancreatic islets in culture.