Calcium currents through voltage-dependent Ca2+channels (VDCCs) in mammalian outer hair cells (OHCs) are generally considered to possess the pharmacological properties of L-type (dihydropyridine-sensitive) currents. However, the OHCs' low resting potentials and their slight depolarization upon sound stimuli suggest that the low voltage-activated channels may contribute to Ca2+regulation. We present morphological and electrophysiological evidence for the presence of the CaV3.1 T-type Ca2+channels, one of the low voltage-activated Ca2+channels, in mature rat OHCs. PCR experiments revealed the expression of CaV3.1, but not CaV3.2 or CaV3.3, in the mature rat cochlea. In situ hybridization and immunohistochemistry revealed expression of CaV3.1 in both inner and outer hair cells at the mRNA level, but only in the OHCs at the protein level. Western blot analysis of anti-CaV3.1 antibody showed a 242 kDa band in mature rat cochlear lysates. Patch-clamp recordings of OHCs isolated from rat cochleae after the onset of hearing revealed that whole-cell voltage-dependent Ca2+ currents were significantly increased in depolarizing steps from a holding potential of -100 mV when compared with those from -70 mV. Only the currents from -100 mV manifested a distinct transient inward Ca2+ current, and this transient component was effectively blocked by 1 microM of the T-type-specific antagonist, mibefradil. Our data suggest an involvement of CaV3.1 in intracellular Ca2+regulation in mature OHCs.