Insulin attenuates vascular contraction via inhibition of voltage-operated Ca²⁺ channels and by enhancement of endothelium-dependent vasodilation. Thus it has been suggested that hypertension-associated insulin resistance results from an insensitivity to the hormone's effects on vascular reactivity. This hypothesis has been strengthened by reports that thiazolidinediones, a class of insulin-sensitizing agents, lower blood pressure and improve insulin responsiveness in hypertensive, insulin-resistant animal models. We tested the hypothesis that troglitazone enhances the vasodilating effect of insulin via inhibition of voltage-operated Ca²⁺ channels in vascular smooth muscle cells. Rat thoracic aortic rings (no endothelium) were suspended in tissue baths for isometric force measurement. Rings were incubated with 0.1 DMSO vehicle (control), troglitazone (10⁵ M), insulin (10⁷ U/l), or both troglitazone and insulin (1 h) and then contracted with phenylephrine (PE), KCl, or BAY K 8644. Troglitazone increased the EC₅₀ values for PE and KCl. Contractions to BAY K 8644 in troglitazone-treated rings were virtually abolished. Insulin alone had no effect on contraction. However, when insulin was combined with troglitazone, the EC₅₀ values for PE and KCl were further increased. Additionally, the maximum contractions to both PE (14 ± 4% of control) and KCl (12 ± 2% of control) were reduced. Measurement of Ca²⁺ concentration ([Ca²⁺]) with fura 2-AM in dispersed vascular smooth muscle cells indicated that neither insulin nor troglitazone alone altered PE-induced increases in intracellular [Ca²⁺]. However, troglitazone and insulin together caused a significant reduction in PE-induced increases in intracellular [Ca²⁺] (expressed as percentage of preincubation stimulation to PE: 47 ± 10%, treated; 102 ± 13%, vehicle). These results demonstrate that troglitazone inhibits Ca²⁺ influx and that it acts synergistically with insulin to attenuate further vascular contraction via inhibition of voltage-operated Ca²⁺ channels.