We have studied the mechanism of Ca current inactivation in the/%cell line HIT-TI5 by conventional and perforated patch recording techniques, using two pulse voltage protocols and a combination of current and tail current measurements. In 5 mM Ca, from a holding potential of -80 mV, the maximum current showed a complex time course of inactivation: a relatively fast, double exponential inactivation (%1 ~ 12 ms and ~h2 ~ 60 ms) and a very slowly inactivating component (z > 1 s). The faster component (%0 was due to the voltage-dependent inactivation of a low-threshold-activated (LVA), T-type current, which deactivates more slowly (z ~ 3-5ms) than the other components (~ ~ 0.2-0.3 ms). The intermediate component (%2) was due to the Ca-dependent inactivation of a portion of the high-threshold-activated (HVA) current. A saturating dose of the dihydropyridine (DHP) nifedipine (10 gM) did not affect the LVA current, but inhibited by 68 _+ 5 % the transient, Ca-sensitive portion of the HVA current and by 33 __+ 12% the long lasting component. We suggest that three components of the calcium current can be resolved in HIT cells and the main target of DHPs is a HVA current, which inactivates faster than the DHP-resistant HVA component and does so primarily through calcium influx.