Nitric oxide (NO), produced by either constitutive or inducible isoforms of NO synthase (cNOS or iNOS), influences myocardial inotropic and chronotropic responses. This pathway has been studied using NO donors or NOS inhibitors or by immune-mediated stimulation of iNOS. Although inhibition of constitutive NO activity in the heart does not influence indices of myocardial contractility, NO donors, in some species and preparations, may exert a negative inotropic effect as well as an enhancement of diastolic relaxation. The best documented cardiac action of NO is inhibition of the positive inotropic and chronotropic responses to beta-adrenergic receptor stimulation. Basal NO production, presumable via cNOS, appears to exert a mild tonic inhibition of beta-adrenergic responses. On the other hand, excessive NO production mediated by iNOS may contribute to the myocardial depression and beta-adrenergic hyporesponsiveness associated with conditions such as sepsis, myocarditis, cardiac transplant rejection, and dilated cardiomyopathy. Muscarinic cholinergic stimulation of the heart appears to stimulate NO production that mediates, at least partially, parasympathetic slowing of heart rate and inhibition of beta-adrenergic contractility. NO-stimulated production of 3',5'-cyclic guanosine monophosphate via guanylyl cyclase accounts for many of the observed physiological actions of NO. 3',5'-Cyclic guanosine monophosphate inhibits the beta-adrenergic-stimulated increase in the slow-inward calcium current and reduces the calcium affinity of the contractile apparatus, actions that could contribute to a negative inotropic effect, an abbreviation of contraction, and an enhancement of diastolic relaxation. Biochemical, immunocytochemical, and molecular biological techniques have been used to show the presence of both cNOS and iNOS within the myocardium. cNOS is expressed in myocytes, endothelial cells, and neurons in the myocardium, and there is evidence for iNOS in myocytes, small vessel endothelium, vascular smooth muscle cells, and immune cells that infiltrate the heart. Taken together, these observations suggest that NO influences normal cardiac physiology and may play an important role in the pathophysiology of certain disease states associated with cardiac dysfunction.