Sex-related differences both in the basal secretion of catecholamines and in the adrenergic reactivity to various stimuli have been described. We studied the responses of catecholamines and arterial blood pressure to clonidine (0.3 mg per os) in 31 normotensive subjects (10 men (M), aged 18-42 years, and 21 women (W), aged 20-48 years). Plasma catecholamines were determined by HPLC at -30, -15, 0, 120, 130, 140 min after clonidine. The basal levels of plasma norepinephrine were similar in men and in women (M = 1.16 +/- 0.26 vs. W = 0.87 +/- 0.07 nmol/l). Basal plasma epinephrine levels were not different in the two sexes (M = 0.21 +/- 0.03 vs. W = 0.14 +/- 0.03 nmol/l). The mean arterial pressure decrease after clonidine was similar in the two groups (M = 13 +/- 3 vs. W = 15 +/- 2 mmHg). The decrease in plasma epinephrine after clonidine was similar in men and women (M = 0.06 +/- 0.04 vs. W = 0.09 +/- 0.02 nmol/l). In contrast, the plasma levels of norepinephrine after clonidine were reduced more in women than in men either when expressed as absolute values (W = 0.63 +/- 0.07 vs. M = 0.3 +/- 0.1 nmol/l; F = 7.6, P < 0.02) or as percentage change (W = 71 +/- 3 vs. M = 34 +/- 8; P < 0.002). The present study demonstrates that an elevated alpha 2-adrenergic activity in women may be responsible for the sexual dimorphism in catecholamine secretion.