Platelet alpha 2-adrenoceptor number was studied in obese subjects treated by a moderate hypocaloric diet (1000-1200 kcal/day = 4200-5000 kg/day) for 21-25 days. Spontaneous lipolysis and adrenergic responsiveness of isolated subcutaneous abdominal adipocytes, collected by a biopsy procedure, were studied before and after treatment on the same subjects. The number of alpha 2-adrenoceptor binding sites (Bmax values) of the platelets was increased after the hypocaloric diet, while there was no change in the equilibrium dissociation constant. Basal lipolytic activity of the adipocytes was increased after the hypocaloric treatment, whereas neither the beta-dependent lipolytic effect of isoproterenol nor the alpha 2-dependent antilipolytic effect of epinephrine were affected. It is concluded that moderate hypocaloric diet induces an up-regulation of platelet alpha 2-adrenoceptors. No modification occurs in alpha 2- and beta-adrenergic responsiveness of the subcutaneous adipocytes. Although alpha 2-adrenergic receptor binding studies on platelets have been widely used as an index for alpha 2-adrenoceptor function in other tissues, it is suggested by this study that platelet alpha 2-adrenoceptors cannot be used for prediction of alpha 2-adrenoceptor function in adipose tissue.