Prostacyclin (PGI 2) dose-dependently increases the adenosine 3′,5′-cyclic monophosphate (cyclic AMP) levels in canine femoral, carotid, and canine and bovine coronary arteries. The prostacyclin-stimulation is enhanced by phosphodiesterase inhibitors, and is readily measurable after 60 sec incubation. The prostaglandin endoperoxide PGH 2, but not PGH 1, also elevates cAMP levels in femoral arteries. Inhibition of arterial prostacyclin synthetase with 28 μM 9,11-azoprosta-5,13-dienoic acid (azo analog I) blocks the PGH 2-stimulation of cAMP accumulation. Azo analog I does not attenuate a direct PGI 2 stimulation, indicating that the PGH 2 dependent elevation of cAMP is due to conversion of PGH 2 to PGI 2 by the artery. PGI 2 and PGE 1 increase cyclic AMP levels and relax dog femoral and bovine coronary arteries, while PGE 2, which actually contracts bovine coronary arteries, has no effect on arterial cyclic AMP levels. The significance of the PGI 2-stimulation of arterial cyclic AMP is not known, but it is probably related to relaxation of arterial strips.