The involvement of protein kinase C (PKC) isoforms in glucose-induced insulin secretion was investigated by comparing the effects of the PKC inhibitors Gö 6976, which is PKC-specific and selective for the Ca²⁺-dependent isoforms, and Ro 31-8220, a specific PKC inhibitor which does not discriminate between isoforms. Gö 6976 inhibited the Ca²⁺- and diacylglycerol (DAG)-dependent PKC activities in β-cell extractsin vitroand fully inhibited insulin secretory responses of rat islets of Langerhans to the PKC activator 4β phorbol myristate acetate (PMA), suggesting that it was an effective inhibitor of the DAG-dependent isoforms of PKCin situ.However, glucose-induced insulin secretion from rat islets was not inhibited by Gö 6976, whereas secretory responses to glucose were partially inhibited by the non-isoform selective PKC inhibitor, Ro 31-8220. The simplest explanation of these results is that glucose-induced insulin secretion is dependent, at least in part, upon the activation of an atypical isoform(s) of PKC within the β-cell.