The red cell adrenoceptor affinity for the unspecific agonists adrenaline and noradrenaline and the specific beta-agonist isoprenaline was studied in vitro on whole blood of rainbow trout, Salmo gairdneri at 15 degrees C. The erythrocytic adrenoceptors could be pharmacologically characterized as beta-receptors of the 'noradrenaline'-type (beta 1-type), with an order of potency of isoprenaline greater than noradrenaline much greater than adrenaline. The adrenoceptor affinities, expressed as agonist concentrations for 50% response (EC50), were 1.3 X 10(-8) and 7.6 X 10(-7) mol l-1 for noradrenaline and adrenaline, respectively. Winter fish showed a red cell adrenergic response identical to that of summer-acclimated fish. It is concluded that most red cell beta-adrenergic responses in vivo are exclusively elicited by noradrenaline.