The O2 consumption of rat pancreatic islets was determined by monitoring pO2 in the perifusate from groups of 200-300 islets. Basal respiration was maintained for up to 2 h. The insulin secretagogues, glucose and 4-methyl-2-oxopentanoate, provoked an immediate (< 5 s) increase in islet respiration which attained a new steady-state within 10-40 min. The respiratory changes were immediately reversible upon removal of the substrate and were paralleled by changes in insulin release and substrate oxidation. The concentration dependence of glucose-induced respiratory changes was sigmoidal with a threshold at 3 mmol/l. The concentration dependence with 4-methyl-2-oxopentanoate was characterised by a hyperbolic relationship. The weak insuln secretagogues 3-methyl-2-oxobutyrate and d,l-3-methyl-2-oxopentanoate, although stimulating islet respiration were not more effective than 4- methyl-2-oxopentanoate at non-insulinotropic concentrations. Rotenone, antimycin and oligomycin inhibited both basal O2 consumption and the ability of glucose and 4-methyl-2-oxopentanoate to increase islet respiration. 2,4-Dinitrophenol increased islet O2 consumption. The omission of Ca2+ and Mg2+ from the perifusing media, or the addition of the ionophore A23187, had little effect on respiration. The omission of K+ inhibited glucose-induced changes but had a lesser effect in the absence of substrate or in the presence of 4-methyl-2-oxopentanoate. The omission of HCO3-reduced both basal and secretagogue-induced changes in islet respiration. It is concluded that mitochondrial O2 consumption linked to oxidative phosphorylation is a major component in the respiratory response, and that some energy consuming process in the islets depends on the availability of HCO3-. Mitochondrial reactions may generate a signal initiating the secretory process.