Immunosuppression is currently used for allotransplantation, and is being evaluated for the treatment of insulin-dependent diabetes mellitus and other autoimmune diseases. However, most available agents have a number of side effects that limit their use in clinical situations. It has been shown previously, for example, that cyclosporine may inhibit insulin release from islet tumor cells and rat islets. We have studied the effects of an analog of a newer agent (FK506) termed L-683,590 on insulin secretion by an islet tumor line, beta TC3, and rat islets, and compared the effects of this drug to those of cyclosporine, since both cause similar immunosuppression. L-683,590 and cyclosporine inhibited insulin release by beta TC3 cells by about 50% and 80%, respectively, at doses that inhibit lymphokine production by T cells. The inhibition by L-683,590 and cyclosporine was more pronounced at higher glucose levels, and was not simply attributable to a general toxic effect of the drugs on the cells. Insulin release during long-term (> 48 hr) cultures of isolated rat islets was also inhibited by the drugs. However, there was no effect of either agent on insulin release by islets during the first 4 hr following a glucose stimulus. Both drugs caused reduced levels of insulin mRNA (by 56 +/- 8.1% and 66 +/- 16% in the presence of L-683,590 and cyclosporine, respectively), accounting for reduced rates of insulin biosynthesis that were also seen. Our studies indicate that: (1) both cyclosporine and L-683,590 inhibit insulin release by beta TC3 cells and cultured rat islets after 48 hr (cyclosporine is a more potent inhibitor); (2) neither drug inhibits the release of insulin during the first 4 hr following a glucose stimulus; and (3) their mechanisms of action appear to be similar--both drugs cause reduced levels of insulin mRNA. Although the toxicity of FK506 on human islets in vivo is still unknown, it may be of particular importance in individuals with impaired beta cell function, such as patients with new-onset insulin-dependent diabetes or patients with non-insulin-dependent diabetes mellitus.