Recent studies from several laboratories have shown perturbations of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] metabolism in hypertension. While these perturbations may exert their effect on blood pressure via their actions on calcium metabolism, it is possible that this vitamin D metabolite may have direct effects on vascular smooth muscle cell (VSMC) physiology. To examine this, we studied the effect of 1,25(OH)2D3 on VSMC growth and found that this substance suppressed VSMC [3H]thymidine uptake; furthermore, this vitamin D metabolite also suppressed the stimulatory effect of epidermal growth factor (EGF) on VSMC proliferation. The concomitant presence of this substance appeared to be required for its action on VSMC growth since cells pretreated with the vitamin D metabolite for up to 72 hours and then washed of the substance grew normally and responded to EGF. Studies were also done to determine if 1,25(OH)2D3 had any effect on the function of EGF receptors on VSMC. Experiments using Iodine-125-labeled EGF showed no differences in the binding of this ligand to VSMC, either untreated or treated with 1,25(OH)2D3, which indicates the effect of the vitamin D metabolite on VSMC growth (when exposed to EGF) was not mediated by an alteration of EGF receptor function. The results of these studies have implications for the pathogenesis of vascular diseases such as hypertension and atherosclerosis.