The effects of physiological levels of 3-hydroxybutyrate on noradrenaline binding to rat erythrocyte membranes were studied. The binding of noradrenaline to membranes was determined by measuring the union of 3H-noradrenaline in the presence of varying concentrations of 3-hydroxybutyrate. Scatchard plots of the results were used to determine that 3-hydroxybutyrate provoked a loss of affinity of the beta 2-adrenergic receptors for adrenaline. Thus high (but physiological) 3-hydroxybutyrate levels can lower the binding of the catecholamine, thus diminishing the extent or intensity of adrenergic-driven responses.