1. Ca2+ current through voltage-dependent Ca2+ channels (ICa) and intracellular free Ca2+ concentration ([Ca2+]i) were measured simultaneously in rat portal vein smooth muscle cells using conventional whole-cell voltage clamp technique and high temporal resolution microfluorimetry. 2. The relationship between depolarization-evoked ICa and rise in [Ca2+]i was examined. The extracellular Ca2+ concentration dependence and the voltage dependence of the depolarization-evoked increases in ICa and [Ca2+]i were similar. Both ICa and increased [Ca2+]i were blocked to a similar extent by nimodipine and cadmium and augmented by Bay K 8644. Furthermore, the time course of the measured increase in [Ca2+]i, closely followed the increase in [Ca2+]i expected from the time-integrated ICa. These observations suggest that the depolarization-evoked rise in [Ca2+]i was tightly coupled to ICa. 3. The cytosolic Ca2+ buffering capacity, determined as the ratio of the expected increase in [Ca2+]i (from ICa) divided by the measured increase in [Ca2+]i, was over 100. Therefore, less than 1 out of 100 Ca2+ ions entering the cell appears as a free Ca2+. 4. Ryanodine (30 microM), a blocker of the Ca(2+)-induced Ca2+ release mechanism, had little effect on buffering capacity measured over the first 200 ms of the depolarizing voltage clamp pulse. Ryanodine also had little effect on the buffering capacity during 800-1000 ms of the depolarizing voltage clamp pulse. Therefore, it was concluded that there is little Ca(2+)-induced Ca2+ release from the stores in rat portal vein smooth muscle cells during depolarization-evoked Ca2+ entry. 5. During brief depolarizations, the largest [Ca2+]i increase and ICa occurred at 0 mV. However, during steady-state depolarization, the largest increase in [Ca2+]i occurred around -30 mV, and we estimate the peak steady-state ICa to be about 0.6 pA.