The influence of GLP-1 on electrical activity and ion currents of mouse pancreatic B-cells was studied with intracellular microelectrodes and the whole-cell configuration of the patch-clamp technique. In the presence of 15 mmol/l glucose 5, 50 and 100 nmol/l GLP-1 slightly increased electrical activity. This effect may be caused by the slowing of Ca2+ channel inactivation observed with GLP-1. Thus, changes in Ca2+ channel kinetics are suggested to contribute to the insulinotropic action of the hormone. The most prominent effect of GLP-1 on the membrane potential was the conversion of irregular electrical activity into regular oscillations of the membrane potential. At the threshold concentration for insulin secretion (7 mmol/l glucose) GLP-1 did not alter the membrane potential. Accordingly, in patch-clamp experiments GLP-1 had no effect on the whole-cell K+ATP current.