Pancreatic islets were isolated from rats that had been nursed by dams fed with a control or an 8.7% protein diet during the first 12 d of the lactation period. Glucose-induced insulin secretion from islets in the 8.7% protein group was reduced 50%. The islet insulin and DNA content were similar, whereas the pancreatic insulin content was reduced by 30 % in the rats fed 8.7 % protein. In order to elucidate the mechanism responsible for the attenuation of insulin secretion, measurements were performed of the activity of several islet enzymes that had previously been supposed to be involved in the coupling of glucose stimulation to insulin secretion. Islet glucose oxidation was unaffected, but glucose-stimulated hydrolysis of phosphatidylinositol was reduced by one-third in the islets of rats fed 8.7% protein. The activity of mitochondrial glycerophosphate dehydrogenase was similar in islets of rats fed the 8.7% protein diet and those fed the control diet. The activity of Ca-independent phospholipase A2 was increased fourfold in the islets of rats fed 8.7% protein. It is concluded that impairment of glucose-induced insulin secretion in rats fed a low-protein diet may be caused by attenuation of islet phosphatidylinositol hydrolysis, and it is tentatively suggested that the increased activity of Ca-independent phospholipase A2 in islets of rats fed a low-protein diet may participate in the stimulation of apoptosis.