Inward currents were measured in cultured rat pancreatic islet cells using the whole-cell patch-clamp technique. Depolarization elicited both transient and more sustained inward currents. The transient current was blocked by external Na removal, TTX or depolarizing holding potentials while the sustained current was blocked by external Ca removal or the addition of Co, Ni or Cd ions to the external solution. Replacing Ca with Ba increased the amplitude of the sustained current and decreased its rate of decay. These results suggest that the transient current was due to the activation of Na channels while the sustained current was mediated by Ca channels. Two characteristics of the data suggested that two Ca currents may contribute to the sustained current. First, a second shoulder, or hump, was observed on the descending limb of the Ca current I-V of many cells, suggesting that they may possess two Ca channels that activate at different voltages. Second, the low and high voltage components of the I-V were differentially suppressed by Cd. Neither of the Ca current components was very sensitive to the Ca channel agonist BAY K 8644. These results were confirmed in HIT cells, another insulin-secreting preparation.