The effects of the four nitro-compounds nitroglycerin, nitroprusside-Na, NaNO2 and B 744-99 were studied simultaneously on length and on cGMP-levels in isolated circular strips of bovine coronary arteries. 1. All 4 nitro-compounds concentration dependently relaxed the strips in close association with pronounced increases in cGMP-levels which preceded the mechanical responses. 2. The relaxant effects of all 4 nitro-compounds were significantly potentiated by the predominant inhibitor of cGMP-hydrolysis M & B 22,948, which also potentiated the increase in cGMP-levels of the two nitro-compounds in which it was studied (nitroglycerin and nitroprusside-Na). 3. Non-substituted cGMP and -- much stronger -- its 8 bromo-derivative also relaxed the strips and these effects were likewise potentiated by M & B 22,948. 4. When the log increase in cGMP produced by the 4 nitro-compounds were plotted against percent relaxation (probit scale) a linear and highly significant positive correlation was obtained. 5. The results provide evidence that the increases in cGMP caused by the 4 nitro-compounds studied are responsible for the smooth muscle relaxing actions of these drugs.