Two inhibitors of the nucleotide-sensitive K+ (KATP) channel, tolbutamide and quinine, were utilized in order to assess the role of this channel in glucose-stimulated insulin release from perifused rat islets. In the absence of these drugs, the addition of 15 mM glucose elicited a marked biphasic stimulation of insulin secretion concomitant with a reduction in the rate of 86Rb+ efflux. In the presence of either 500 microM tolbutamide or 100 microM quinine, a reduced rate of efflux of 86Rb+ was observed together with an elevated rate of insulin release. Under such conditions, the addition of 15 mM glucose retained the ability to stimulate insulin secretion though this was associated with a marked increase in 86Rb+ efflux. It is concluded that a net reduction in beta-cell K+ permeability is not an obligatory step in glucose-stimulated insulin release. Thus, glucose is likely to exert depolarizing actions on the beta-cell in addition to the closure of K+ channels.