In this study we investigated if sodium nitroprusside (NaNP), a vasodilator with direct action on smooth muscle, would have any effect on calcium kinetics in blood vessels. The effect of NaNP, in 0.1-10 micrometer concentration, was studied in vitro on the uptake of 45Ca with the lanthanum method and on the efflux of 45Ca. The drug failed to alter the uptake of 'La-resistant' 45Ca in rabbit mesenteric artery and vein. This was true even if Ca2+ uptake was increased in the vessels by 60 mM KCl. The efflux of 45Ca on the other hand was enhanced from rabbit mesenteric vein and from rat aorta exposed to NaNP, in a dose-related way. The present study, like our previous one on other vasodilators, indicates that NaNP has en effect on vascular calcium kinetics but this is the first time that a relationship could be established between the dose of the drug and the effect on 45Ca efflux.