An inhibitor of adenylate and guanylate cyclases was tested on strips of left atria from rabbits. Effects of catecholamines (cardiotonic) and of acetylcholine (cardiodepressive) were blocked, and positive force-frequency was converted to negative. Ouabain produced only contracture without positive inotropy. The cardiotonic effect of increased calcium remained. Data suggest that cyclic nucleotides modulate calcium associated with these stimuli.