1. Methylxanthines have been shown to elevate the basal plasma level and/or urinary excretion of noradrenaline (NA) and adrenaline (ADR) in healthy subjects. The present study addressed the hypothesis that the methylxanthine aminophylline also augments plasma and urinary catecholamines during increased sympathoadrenal activity. 2. Eleven healthy young men performed a maximal 2 h bicycle exercise twice, after double-blind intravenous administration of placebo or aminophylline. Femoral venous plasma and urinary concentrations of NA and ADR were analysed in samples representing basal state, exercise and recovery, using liquid chromatography with electrochemical detection. 3. Leg exercise induced eight- and six-fold increases in the plasma concentrations of NA and ADR, respectively, and seven- and four-fold increases in the urinary concentrations of NA and ADR, respectively, indicating that sympathoadrenal activity was considerably elevated. 4. After aminophylline (mean plasma concentration 20–35 μmol/l), the plasma concentrations of NA ( P < 0.001) and ADR ( P < 0.05) were independently higher at rest, during exercise and during recovery, in comparison to after placebo; the mean exercise plasma level of NA was increased by the drug from 13 ± 1 to 21 ± 2 nmol/l and the corresponding level of ADR from 2.1 ± 0.4 to 2.9 ± 0.5 nmol/l. Also urinary NA ( P < 0.01) and ADR ( P < 0.05) were elevated by aminophylline; the exercise concentrations of NA in the urine were 75 ± 8 and 97 ± 10 μmol/mol of creatinine after placebo and aminophylline, respectively, and the corresponding levels of ADR were 12 ± 3 and 16 ± 3 μmol/mol of creatinine, respectively. 5. These data indicate that the facilitating effect of methylxanthines on plasma and urinary NA and ADR is present also when the sympathoadrenal activity is elevated by physical exercise.