Pancreatic islet B cells depolarize and display trains of action potentials in response to stimulatory concentrations of glucose. Based on data from rodent islets these action potentials are considered to be predominantly Ca2+ dependent. Here we describe Na(+)-dependent action potentials and Na+ currents recorded from canine and human pancreatic islet B cells. Current-clamp recording using the nystatin "perforated-patch" technique demonstrates that B cells from both species display tetrodotoxin-sensitive Na+ action potentials in response to modest glucose-induced depolarization. In companion "whole-cell" voltage-clamp experiments on canine B cells, the underlying Na+ current displays steep voltage-dependent activation and inactivation over the range of -50 to -40 mV. The Na+ current is sensitive to tetrodotoxin block with a KI = 3.2 nM and has a reversal potential which changes with [Na+]o as predicted by the Nernst equation. These results suggest that a voltage-dependent Na+ current may contribute significantly to action potential generation in some species outside the rodent family.