The evidence for the involvement of protein kinase C (PKC) in insulin secretion stimulated by glucose and Ca2(+)-mobilizing receptor agonists has been reviewed. Results of phorbol ester binding to intact cells and the measurements of the proportion of PKC associated with the membrane after cell fractionation are presented. Glucose stimulation leads to increased phorbol ester binding without causing membrane insertion of the enzyme which, however, occurs with receptor agonists. It is suggested that the rise in cytosolic Ca2+ in response to glucose favours the apposition of PKC to the membrane whereas intercalation of the enzyme requires phospholipase C-mediated generation of diacylglycerol. It is possible that this effect of glucose on PKC, although not involved in the initiation of secretion, could explain the potentiation of insulin release observed in the presence of the receptor agonists.