1. When pancreatic islets were exposed to a K(+)-free medium, the intracellular concentration of K(+) was decreased and that of Na(+) increased. 2. In the K(+)-deprived islets, the utilization of [5-(3)H]glucose, output of lactic acid and oxidation of [U-(14)C]-glucose were decreased by about 30-40% below the control values found at normal extracellular K(+) concentration (5.0mm). However, the oxidation of [U-(14)C]pyruvate was unaffected. 3. The omission of extracellular K(+) little affected the production of (14)CO(2) from islets prelabelled with [U-(14)C]palmitate and incubated in the absence of glucose, despite the fact that K(+) deprivation significantly increased the ATP concentration and ATP/ADP concentration ratio in the glucose-deprived islets. 4. At normal K(+) concentration, glucose increased the concentrations of phosphoenolpyruvate, NAD(P)H and ATP in the islets. In the glucose-stimulated islets, the concentration of phosphoenolpyruvate, but not that of either NAD(P)H or ATP, was higher in the absence than in the presence of extracellular K(+). In islet homogenates, the activity of pyruvate kinase (EC 2.7.1.40) was stimulated by K(+) (optimal activity at 100-150mm-K(+)) and inhibited by Na(+) (except at very low K(+) concentrations). 5. K(+) could be replaced by NH(4) (+), Rb(+), Cs(+) or Na(+) to maintain, at least to some extent, pyruvate kinase activity in islet homogenates. Addition of Rb(+) or Cs(+), but not NH(4) (+), to K(+)-deprived media also increased [U-(14)C]glucose oxidation by intact islets. 6. The omission of K(+) did not cause any obvious anomaly in the apparent dependency of (45)Ca(2+) net uptake on NAD(P)H concentration in the islets. 7. These data suggest that the coupling between metabolic and ionic events in the islet cells involves feedback mechanisms through which glucose oxidation may be modulated by cationic factors.