Plasma and tissue norepinephrine (NE) concentrations were determined at rest and after 45 min of swimming in rats with a surgically induced myocardial infarction (MI) and in rats having undergone a sham operation (SHAM). The MI rats had moderate-sized infarcts and demonstrated decreases in maximal O2 uptake (VO2max) that are consistent with the contention that the animals possessed a significant amount of left ventricular (LV) dysfunction and chronic heart failure (CHF). Plasma NE concentrations measured at rest were not significantly different between the SHAM and MI groups of rats, although a strong trend was found for the plasma NE concentrations to be elevated in the MI group. The plasma NE responses to 45 min of swimming at the same absolute submaximal workload were similar in the two groups of rats in light of the fact that the MI group of rats exercised at a greater percentage of their VO2max when compared with their SHAM counterparts. Exercise produced significant reductions in the NE concentrations of the diaphragm, vastus lateralis, red portion of the gastrocnemius, plantaris, and vastus intermedius muscles for both the SHAM and MI groups of rats. In addition, the NE concentrations measured in both the soleus and red portion of the gastrocnemius muscle were significantly greater in the MI rats when compared with their SHAM counterparts for both rest and exercise conditions. The results from the present study support the hypothesis that the sympathetic response to exercise is either unchanged or attenuated in MI rats that have a significant amount of LV dysfunction and CHF.(ABSTRACT TRUNCATED AT 250 WORDS)