Serum catecholamine levels and myocardial and lymphocyte adrenergic receptor (AR) concentrations were measured in adult great danes affected by canine dilated cardiomyopathy (DCM) and compared to those of healthy animals. A non-homogeneous population of β -AR, consisting of β 1 -AR and β 2 -AR, was observed in healthy (41 and 59%, respectively) and affected (17 and 83%, respectively) dog lymphocytes. Binding assays revealed that total β -AR, β 1 -AR and α 1 -AR were significantly downregulated (P<0.05;P<0.01;P<0.001), both in lymphocyte and myocardial cell membranes of affected dogs. β 2 -Adrenergic receptor concentrations were significantly reduced only in lymphocyte and right atrium cell membranes (P<0.05). Downregulation was not associated with alterations in receptor binding characteristics, as no significant differences in K d values were found. Mean plasma catecholamine levels were significantly higher (P<0.01) in DCM dogs (939±41) than in normal subjects (348±32), thus suggesting a sympathetic activation. The present study indicates a condition similar to that observed in human patients affected by DCM and that adrenergic receptors in canine lymphocytes reflect the fluctuation of adrenergic receptor concentrations in the myocardium.