The effects of obesity and gender on plasma catecholamine levels were studied in 10 lean men, 10 lean women, and 10 obese women. Measurements (mean +/- SE) were obtained sequentially after 30 min of bedrest, 10 min of standing, and 5 min of sustained handgrip, and following 15 min of rest while standing. The supine plasma norepinephrine (NE) levels were comparable among the 3 groups, whereas plasma epinephrine (E) was lowest in obese women (14 +/- 2 pg/ml), intermediate in lean women (19 +/- 2 pg/ml), and highest in lean men (29 +/- 5 pg/ml). Peak plasma levels were reached during handgrip. E values in men (139 +/- 27 pg/ml) exceeded those in lean women (71 +/- 9 pg/ml; P less than 0.05), and both were higher than the E response in obese women (38 +/- 7 pg/ml; P less than 0.01). While peak plasma NE levels in lean women (654 +/- 62 pg/ml) and obese women (524 +/- 46 pg/ml) were comparable, both were significantly lower than the NE response in lean men (1014 +/- 114 pg/ml; P less than 0.02). Increments in plasma FFA during handgrip were 21% and 28% above values during standing (P less than 0.005) in lean men and lean women, respectively, whereas no net increment occurred in obese women. The excursion of plasma beta-hydroxybutyrate in recovery was 59% above the standing concentration for lean women (P less than 0.05), but no net increments were observed in obese women or lean men. We conclude that obesity in women is attended by suppressed plasma E responses to isometric exercise, which may explain subnormal excursions of plasma FFA and beta-hydroxybutyrate during handgrip and recovery, respectively. Lean men have greater plasma elevations of E and NE during isometric exertion than women, suggesting an influence of sex on plasma catecholamine regulation.