1. The present experiment was undertaken to investigate: (a) the effect of nitric oxide synthase (NOS) inhibition, mediated by oral supplementation of the NOS inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on measures of myocardial energy metabolism and function: (b) the effect of oral creatine supplementation on these variables, in the absence and presence of L-NAME. 2. In one series of experiments, 4 weeks oral administration of L-NAME (0.05 mg ml-1 day-1 in the drinking water) to Brattleboro rats caused significant reductions in myocardial ATP, creatine, and total creatine concentrations and an accumulation of tissue lactate when compared with control animals. Administration of creatine (0.63 mg ml-1 day-1 in the drinking water) for 4 weeks elevated myocardial creatine and total creatine concentrations and reduced lactate accumulation, but did not significantly affect ATP or phosphocreatine (PCr). Concurrent treatment with creatine and L-NAME prevented the reduction in creatine and total creatine concentrations, and significantly attenuated the accumulation of lactate and the reduction in ATP seen with L-NAME alone. 3. In a second series of experiments, 4 weeks treatment with L-NAME and creatine plus L-NAME increased mean arterial blood pressure in conscious Brattleboro rats. Hearts isolated from these animals showed decreased coronary flow and left ventricular developed pressure (LVDP), and total mechanical performance. Treatment with creatine alone had no measurable effect on either mean arterial blood pressure or coronary flow in isolated hearts. However, there was an increase in LVDP, but not in total mechanical performance, because there was a bradycardia. 4. These results indicate that creatine supplementation can attenuate the metabolic stress associated with L-NAME administration and that this effect occurs as a consequence of the action of creatine on myocardial energy metabolism.