1. The actions of troglitazone, pioglitazone, metformin and bezafibrate, agents that improve insulin-resistance, on voltage-dependent Ca2+ channels in arterial smooth muscle cells were examined by use of the conventional and nystatin-perforated whole-cell clamp methods. Single cells were freshly isolated from resistance mesenteric arteries of guinea-pigs. The actions of these agents on 77 mM K+-induced contraction of the isolated arteries were also examined with the use of isometric tension recording. 2. The thiazolidinedione derivatives, troglitazone and pioglitazone, inhibited whole-cell Ca2+ currents in a dose-dependent manner with dissociation constants of 3.0 microM and 44.9 microM and Hill coefficients of 0.61 and 0.68, respectively. These two agents inhibited the 77 mM K+-induced contraction with similar potencies as those inhibiting the Ca2+ currents. Metformin and bezafibrate had no apparent effects on the Ca2+ current or high K+-induced contraction. 3. The inhibitory action of troglitazone on Ca2+ currents was not affected by the command potential, the holding potential, or the stimulation frequency, suggesting that its mode of the action differs from that of known organic Ca2+ channel antagonists. 4. The inhibitory action of troglitazone on Ca2+ currents was not affected by the addition of insulin to, or the removal of glucose from, the solutions. 5. In conclusion, the thiazolidinedione derivatives directly inhibited the voltage-dependent Ca2+ channels in a different manner from that of organic Ca2+ channel antagonists. This inhibitory action on Ca2+ channels was not a common feature of insulin-sensitizing agents.