We have investigated the effect of atrial natriuretic hormone on serum-induced mitogenesis in cultured rat mesangial cells. Synthetic peptides, atriopeptin 28 and atriopeptin 24, dose-dependently decreased thymidine incorporation, with a half-maximal effect at ∼1 nM and a maximal inhibition of ∼60%. Moreover, atriopeptin 28 significantly decreased the clonal proliferation of mesangial cells. Atriopeptin 28 also decreased resting cytosolic Ca but had no effect on the increase induced by serum, relative to the lower baseline established by atriopeptin 28. Nevertheless, the overall effect of atriopeptin 28 on Ca was to attenuate the serum-induced increase, relative to the original resting level. These results therefore provide evidence for a novel biological effect of atrial natriuretic hormone and suggest that the antimitogenic effect may be mediated by atriopeptin-induced alterations of intracellular Ca dynamics. We speculate that atrial natriuretic hormone may be a modulator of mesangial cell mitogenesis in vivo.