Hypothermia increases the sensitivity of isolated cardiac muscle to stimulation by beta-adrenoceptor agonists. The purpose of this study was to determine pharmacologically whether this supersensitivity is associated with a change in the affinity of agonists for the receptor. The positive inotropic and chronotropic responses of guinea-pig paced left and spontaneously beating right atria were recorded. Cumulative dose--response curves to noradrenaline (or adrenaline) were compared with isoproterenol in each tissue. At 38 degrees C, the rate curves were to the left of the tension curves, with lower mean effective concentration (EC50) values. However, this difference was less for noradrenaline and adrenaline which were therefore tension selective relative to isoproterenol. Lowering the temperature to 25 degrees C induced supersensitivity, all dose--response curves being displaced to the left. In the presence of carbachol the curves were shifted to the right with depression of the maxima. Dissociation constants (KA) were calculated from plots of reciprocals of equiactive concentrations obtained before and in the presence of carbachol. KA values for rate and tension responses of each agonist were identical at 38 degrees C, indicating that the rate selectivity was not due to affinity differences. The efficacies (er) of noradrenaline and adrenaline were greater than isoproterenol for tension, but smaller for rate responses, which may explain their relative tension selectivity. At 25 degrees C, the KA values of all agonists were reduced approximately 10-fold. Hypothermia-induced supersensitivity is therefore associated with an increase in affinity for the cardiac beta-adrenoceptor.