Voluntary heart rate (HR) control during moderate exercise on a bicycle ergometer was studied in 10 healthy physically conditioned men (5 experimental and 5 control). The results showed that subjects could learn to attenuate the tachycardia of exercise while exercising at a steady work level of 60-70% of maximum HR. Experimental subjects who saw beat-to-beat displays of HR and were instructed to slow HR showed 22% less increase in HR than did control subjects who exercised without HR displays or instruction to slow HR (42.6 vs. 54.6 beats/min). When the control subjects were given feedback in additional sessions, they also decreased HR significantly by 9% (54.6 vs. 49.9 beats/min). Analyses of concomitant respiratory and metabolic data showed that HR attenuation was accompanied by decreased O2 consumption (P less than 0.06) and pulmonary ventilation (P less than 0.01). Rate pressure product also fell, indicating a decrease in myocardial O2 consumption. Comparisons among pre- and postsubmaximal and cardiovascular pulmonary and humoral responses during maximal test sessions suggested that the improvement in cardiopulmonary function during feedback training occurred with no sacrifice to working muscle requirements because blood lactate concentrations were similar. The attenuation of the HR response obtained in the present study indicates that feedback training in physically conditioned subjects can influence cardiovascular responses even under conditions of heavy local demands imposed by working muscles.