Vascular resistance and arterial pressure are reduced during normal pregnancy, but dangerously elevated during pregnancy-induced hypertension (PIH), and changes in nitric oxide (NO) synthesis have been hypothesized as one potential cause. In support of this hypothesis, chronic inhibition of NO synthesis in pregnant rats has been shown to cause significant increases in renal vascular resistance and hypertension; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the pregnancy-associated changes in renal vascular resistance reflect changes in contractility and intracellular Ca²⁺ concentration ([Ca²⁺]_i) of renal arterial smooth muscle. Smooth muscle cells were isolated from renal interlobular arteries of virgin and pregnant Sprague-Dawley rats untreated or treated with the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 4 mg · kg¹ · day¹ for 5 days), then loaded with fura 2. In cells of virgin rats incubated in Hanks' solution (1 mM Ca²⁺), the basal [Ca²⁺]_i was 86 ± 6 nM. Phenylephrine (Phe, 10⁵ M) caused a transient increase in [Ca²⁺]_i to 417 ± 11 nM and maintained an increase to 183 ± 8 nM and 32 ± 3% cell contraction. Membrane depolarization by 51 mM KCl, which stimulates Ca²⁺ entry from the extracellular space, caused maintained increase in [Ca²⁺]_i to 292 ± 12 nM and 31 ± 2% contraction. The maintained Phe- and KCl-induced [Ca²⁺]_i and contractions were reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. Phe- and KCl-induced contraction and [Ca²⁺]_i were not significantly different between untreated and L-NAME-treated virgin rats or between untreated and L-NAME + L-arginine treated pregnant rats. In Ca²⁺-free Hanks', application of Phe or caffeine (10 mM), to stimulate Ca²⁺ release from the intracellular stores, caused a transient increase in [Ca²⁺]_i and a small cell contraction that were not significantly different among the different groups. Thus renal interlobular smooth muscle of normal pregnant rats exhibits reduction in [Ca²⁺]_i signaling that involves Ca²⁺ entry from the extracellular space but not Ca²⁺ release from the intracellular stores. The reduced renal smooth muscle cell contraction and [Ca²⁺]_i in pregnant rats may explain the decreased renal vascular resistance associated with normal pregnancy, whereas the enhanced cell contraction and [Ca²⁺]_i during inhibition of NO synthesis in pregnant rats may, in part, explain the increased renal vascular resistance associated with PIH.