The central nervous system myelin basic protein (MBP) stimulates the release of several peptide hormones including insulin and glucagon. This could be associated with the development of hyperglycaemia in neurological disorders such as stroke, in which MBP is known to leak into blood circulation. In the present study the mechanism of insulin and glucagon release was investigated by using short-term incubation of isolated rat pancreatic islets. Incubation with MBP in the absence of Ca2+ resulted in approx. 11-fold stimulation of insulin and glucagon release. The stimulation dwindled with increasing Ca2+ concentration and was 6.5-fold at 0.5 mM and 2-fold at 2.5 mM Ca2+. When MBP and glucose at various concentrations were simultaneously present in the incubation mixture, stimulation of insulin release was the sum of the stimulation induced by these two agents separately both at the 0.5 and 2.5 mM Ca2+ concentrations. Glucose at concentrations of 10 or 15 mM did not suppress MBP-stimulated glucagon release. Caffeine-evoked increase in intracellular Ca2+ was without effect on MBP-stimulated insulin or glucagon release but enhanced glucose-induced insulin release. The Ca2+ channel blocker diltiazem had no effect on MBP-stimulated insulin release at concentrations where glucose-stimulated release was inhibited. Ruthenium red inhibited both MBP- and glucose-stimulated insulin release as well as MBP-induced glucagon release. Staurosporine (inhibitor of protein kinase C) had no effect on MBP-induced insulin release, although it partially inhibited glucose-stimulated release. Maleylation of MBP abolished its insulin- and glucagon-releasing activity by approx. 90%. These results suggest that MBP exerts its insulin-releasing effect by mechanisms different from those of glucose-stimulated insulin release and does not require Ca2+ channels or protein kinase C. The relation of MBP-induced insulin and glucagon release to Ca2+ concentration is probably explained by enhanced self-aggregation of MBP or by increased ability of MBP to interact with islet cell membranes in the absence of Ca2+, or both. It is concluded that MBP-induced hormone release appears to be mediated by membrane fusion and oligomerization of MBP. The mechanism thus resembles that of various toxins and other cytotoxic agents.