Phosphoinositide hydrolysis in intact pancreatic islet cells was investigated in an indirect but dynamic manner by monitoring the efflux of radioactivity from islets prelabelled with [3H]inositol. A rise in glucose concentration provoked a rapid, modest but sustained increase in effluent radioactivity, this phenomenon being abolished in the absence of extracellular Ca2+ or presence of verapamil. The release of [3H]inositol was also stimulated at high extracellular K+ concentration, but not by gliclazide. Whether in the presence or absence of glucose, carbamylcholine provoked a marked increase in effluent radioactivity. The response to the cholinergic agent was decreased in the presence of verapamil or absence of extracellular Ca2+ and abolished in the presence of atropine or LiCl. These results suggest that an increase in cytosolic Ca activity, as caused by glucose or membrane depolarization, may cause activation of phospholipase C. In response to cholinergic agents, however, the enzymic activation, although modulated by Ca2+ availability, may result directly from the occupation of muscarinic receptors.