Smooth muscle cells were isolated from rat portal vein and studied during short-term primary culture using the whole-cell patch-clamp technique. Two distinct types of Ca channel could be separated by studying the inward currents in Ba solutions. The rapidly inactivating current was present when cells were held at very negative potentials (-80 mV). This current was prominent for relatively small depolarizations and was insensitive to nifedipine. A slowly inactivating current, corresponding to the slow Ca current previously reported in smooth muscles, was observed at less negative holding potentials (-50 mV), was prominent for positive depolarizations and was blocked by low concentrations of nifedipine. Both currents were unaffected by tetrodotoxin and both were blocked by Co.