Previous studies have suggested that flatfish lack adrenergic cardiac innervation and have a limited humoral adrenergic stress response. However, data on neurohormonal control of flatfish cardiac function is scarce, and has never been directly studied in vivo. Hence, we (1) injected neural and humoral antagonists into flounder (Pleuronectes americanus) in vivo to determine the contribution of autonomic innervation and circulating catecholamines to the control of resting cardiac function; (2) measured pre- and post-stress (90 s chase) catecholamine levels in this species; and (3) constructed in vivo catecholamine dose-response curves for cardiovascular function based on the results of the second experiment. In addition, we quantified the density (B(max)) and ligand-binding affinity (Kd) of flounder ventricular cell-surface beta-adrenoreceptors, and established whether they were of beta1 or beta2 subtype using pharmacological antagonists. The cholinergic contribution to resting flounder heart rate was comparable to other teleosts (cholinergic tonus 26%). Interestingly, however, bretylium increased heart rate, resulting in a negative resting adrenergic tonus (-11.9%), and we were unable to demonstrate that catecholamines supported cardiac function at rest or at circulating concentrations approximating those following an exhaustive chase (adrenaline, 21 nmol l(-1); noradrenaline, 14 nmol l(-1)). Myocardial B(max) was very high in the flounder (252.8 fmol mg(-1) protein), and it appears that flounder ventricular beta-adrenoreceptors are predominantly of the beta2 subtype [based on the inability of atenolol to displace [3H]CGP from the beta-adrenoreceptors, and the IC(50) value for ICI 118551 (1.91 x 10(-6) mol l(-1))]. However, the extremely low affinity (Kd 1.02 nmol l(-1)) for these receptors raises the possibility that the flounder heart is also populated by beta3-adrenoreceptors.