1. Experiments were designed to examine the effect of cholera toxin and forskolin on the release of relaxing factor(s) from superfused cultured endothelial cells under basal conditions and upon stimulation with bradykinin, adenosine diphosphate or the calcium ionophore A23187. 2. Exposure of cultured porcine aortic endothelial cells to cholera toxin (30 micrograms/ml, for 3 hr) and forskolin (10(-6) M, for 45 min) significantly increased the intracellular content in cyclic AMP. Cholera toxin but not forskolin stimulated the accumulation of cyclic GMP. 3. Exposure to cholera toxin did not modify the basal release of endothelium-derived relaxing factor nor that induced by adenosine diphosphate, but significantly increased that evoked by bradykinin and the calcium ionophore A23187. Forskolin did not significantly affect the basal or the stimulated release of endothelium-derived relaxing factor. 4. These results suggest that cholera toxin potentiates the release of endothelium-derived relaxing factor (presumably nitric oxide) from endothelial cells by a mechanism other than augmented production of cyclic AMP.