We determined if the tonic autonomic nervous system (ANS) contribution to arterial blood pressure (BP) maintenance in humans is related to habitual endurance exercise status. Twenty-three healthy young (age 18-31 years) males, 11 endurance exercise-trained and 12 untrained, were studied. Maximal oxygen consumption was higher (P < 0.001) and resting heart rate and body fatness were lower (P < 0.05) in the exercise-trained men. Plasma noradrenaline concentrations and BP decreased from baseline levels in response to ganglionic blockade (intravenous trimethaphan) in both groups (all P < 0.001). The absolute (Delta mmHg: systolic = -35 +/- 2 vs. -32 +/- 4; diastolic = -13 +/- 2 vs. -10 +/- 2; mean = -21 +/- 2 vs. -17 +/- 3) and relative (Delta%: systolic = -35 +/- 2 vs. -31 +/- 3; diastolic = -26 +/- 3 vs. -20 +/- 3; mean = -31 +/- 2 vs. -26 +/- 3) decreases in BP were not significantly different between the endurance-trained and untrained men. There were no significant group differences in the heart rate, stroke volume, cardiac output or systemic vascular resistance (conductance) responses to trimethaphan. Systemic vascular alpha-adrenergic sensitivity (slope of the increase in mean BP with incremental phenylephrine infusion during ganglionic blockade) also did not differ in the two groups (endurance-trained: 3.2 +/- 0.5; untrained: 3.2 +/- 0.7 mmHg (ng phenylephrine)(-1) (ml plasma)(-1)). In the pooled sample, the decrease in mean BP during trimethaphan was related to baseline and changes in plasma noradrenaline concentrations (r = 0.58-0.65, P < 0.001) and alpha-adrenergic sensitivity (r = 0.49, P < 0.02). Our results suggest that the endurance exercise-trained state is not obviously associated with altered ANS support of BP in healthy young men. Basal sympathetic nervous system (SNS) activity and alpha-adrenergic vascular sensitivity are significant physiological correlates of ANS support of BP in this population.