We compared effects of perfusion of norepinephrine (NE, 10-9 mol 1-1) and of unchanged Krebs-Henseleit solution on the cardiac response to bolus injection of arginine vasopressin (AVP, 2 ng). 14 isolated rat working heart preparations were used in a balanced cross-over design. Coronary flow, oxygen consumption and extraction, heart rate and total flow were continuously recorded. The concentration of NE was below that exerting per se systematic influences on cardiac activity. However, NE changed the cardiac response to AVP: (1) the AVP-induced reduction in coronary flow was greater during NE (mean: 41.7%) than vehicle perfusion (30.5%, P<0.005). (2) The AVP-induced decrease in oxygen consumption was stronger on top of the NE (41.5%) than vehicle perfusion (33.6%, P<0.005). (3) Following AVP, oxygen extraction during NE was increased compared to oxygen extraction during vehicle perfusion (3.61 ± 0.03 vs. 3.46 ± 0.02 μl O2 ml-1 g-1, P<0.005). Results support the view of a potentiating role of catecholamines for direct cardiovascular effects of AVP.