Our aim was to investigate plasma catecholamine responses to so-called 'doping' drugs and exercise in Thoroughbreds. Plasma adrenaline (Ad) and noradrenaline (NA) were determined after the administration of caffeine and promazine hydrochloride (PRZ) using a high performance liquid chromatographic method. Caffeine or PRZ was administered i.m. to Thoroughbreds and its effects on plasma catecholamines at rest and during exercise were compared with the saline control. The treadmill exercise was performed 1 h after administration. A dose of 5.0 mg/kg bwt caffeine was found to significantly increase both plasma Ad and NA levels but this was not the case for the 2.5 mg/kg bwt dose and their peak levels at 1.5 h were about 3 and 2.5 times as compared with the control values at 1.5 h (Ad: mean +/- s.e. 21.2 +/- 2.8 pg/ml, NA: 55.5 +/- 4.1 pg/ml), respectively. Both 1.0 and 1.5 mg/kg bwt PRZ doses reduced the plasma Ad to below the detection limit (10 pg/ml) and significantly reduced the plasma NA. The 2.5 mg/kg bwt caffeine dose significantly increased plasma Ad and NA during exercise and approximately doubled their maximal values as compared with the saline control (Ad: mean +/- s.e. 12.328 +/- 4.733 ng/ml, NA: 9.997 +/- 4.146 ng/ml). The 1.5 mg/kg bwt PRZ dose decreased the plasma Ad during exercise but the effect was not significant. On the other hand, PRZ significantly increased the plasma NA as compared with the saline control. In conclusion, the present study demonstrates that the plasma catecholamine responses to caffeine and PRZ were modified by exercise. It is probable that the modification may be related to exercise-induced activation of the sympathetico-adrenal axis.