Isolated perfused rat pancreases were exposed, in the presence of 10.0 mM L-leucine, to either -D-glucose pentaacetate, -L-glucose pentaacetate, or unesterified D-glucose, all tested at a 1.7 mM concentration. The pentaacetate ester of -D-glucose and, to a lesser extent, that of -L-glucose stimulated both insulin and somatostatin release, whereas unesterified D-glucose failed to do so. In the case of insulin output, the two esters differed from one another not solely by the magnitude of the secretory response but also by its time course and reversibility. Compared with these data, the most salient difference found in the case of somatostatin release consisted of the absence of an early secretory peak in response to -D-glucose pentaacetate administration and the higher paired ratio between the secretory responses evoked by the esters of glucose and by unesterified D-glucose (5.5 mM) administered at the end of the experiments. The two esters provoked an initial and short-lived stimulation of glucagon secretion, in sharp contrast to the immediate inhibitory action of unesterified D-glucose. Thereafter, -D-glucose pentaacetate, but not -L-glucose pentaacetate, caused inhibition of glucagon release, such an effect being reversed when the administration of the ester was halted. These findings indicate a dual mode of action of glucose pentaacetate esters on hormonal secretion from the endocrine pancreas. The intracellular hydrolysis of -D-glucose pentaacetate and subsequent catabolism of its hexose moiety may contribute to the early peak-shaped insulin response to this ester, to the persistence of a positive secretory effect in B and D cells after cessation of its administration, and to the late inhibition of glucagon release. However, a direct effect of the esters themselves, by some as-of-yet unidentified coupling process, is postulated to account for the stimulation of insulin and somatostatin release by -L-glucose pentaacetate and for the initial enhancement of glucagon secretion provoked by both glucose esters.