We used an algebraic model of resting heart rate (HRr), HRr–mn (HRo), to compare resting parasympathetic (n) and sympathetic (m) influence, intrinsic heart rate (HRo), and resting autonomic balance (Au) in ten endurance–trained (ET) and ten nontrained (NT) men. The values of m, n, and AMI were determined by selective pharmacological blockade with atropine and metoprolol. HR,, was obtained during double blockade with atropine and metoprolol. HRO and HRr were significantly lower (P < 0.04 and P < 0.01, respectively) in the ET subjects(79.5 + 2.8 beats·min-land 54.7 ± 3.0 beats·min-1respectively) when compared to the ET subjects (86.6 ± 2.5 beats·min and 70.2 ±3.1 beats·min-1, respectively). Parasympathetic influence in) was greater in the ET subjects (P < 0.04), while sympathetic influence (m) was slightly (P < 0.05) less in the ET subjects. Consequently, the value of Ahai was significantly less in the ET subjects (P < 0.02), indicating that resting parasympathetic predominance was significantly greater in the ET subjects. We concluded that the exercise training bradycardia, observed in this group of subjects, was due to both a lower HRo and an Ata, with an augmented parasympathetic dominance.