The effects of L- and D-phenylisopropyladenosine (L- and D-PIA) were studied on glucose-induced insulin secretion from the isolated perfused rat pancreas. L-PIA at the low dose of 16.5 nM inhibited insulin secretion by 50%. In contrast, D-PIA at 16.5 and 82.5 nM was ineffective. D-PIA used at a 100-fold higher concentration (1.65 microM) than L-PIA induced a similar inhibition of insulin secretion. The inhibitory effect of L-PIA was abolished by 8-phenyltheophylline (1 microM), a potent P1 purinoceptor antagonist. The present experiments provide evidence for an adenosine receptor of the A1 subtype on the insulin-secreting pancreatic cell of rats.