To determine whether venous plasma norepinephrine concentrations consistently reflect changes in sympathetic nervous activity, the influence of mental arithmetic, static handgrip, and submaximal bicycle exercise on intra-arterial blood pressure, heart rate, and plasma norepinephrine was studied in 51 subjects with untreated essential hypertension (mean age, 46 years; range, 16-69 years). At rest, plasma norepinephrine was unrelated to age or blood pressure. Mental arithmetic increased mean arterial pressure from 108 +/- 18 to 127 +/- 18 mm Hg (mean +/- S.D.; p less than 0.001) and heart rate from 69 +/- 7 to 93 +/- 13 beats/min (p less than 0.001) but not plasma norepinephrine (547 +/- 297 to 518 +/- 250 pg/ml). Isometric exercise raised mean arterial pressure from 115 +/- 18 to 148 +/- 21 mm Hg (p less than 0.001) and heart rate from 76 +/- 9 to 95 +/- 13 beats/min (p less than 0.001) but not plasma norepinephrine (683 +/- 253 to 741 +/- 253 pg/ml). Bicycle exercise increased mean arterial pressure from 114 +/- 20 to 146 +/- 26 mm Hg (p less than 0.001), heart rate from 77 +/- 9 to 128 +/- 19 beats/min (p less than 0.001), and plasma norepinephrine from 645 +/- 228 to 1151 +/- 462 pg/ml (p less than 0.001). Both the maximum mean arterial pressure and the peak heart rate attained during bicycle exercise were related to the exercise plasma norepinephrine level (r = 0.33, p less than 0.02 and r = 0.28, p less than 0.03, respectively). Increases in plasma norepinephrine with exercise were not greater in older or more hypertensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)