The effect of chronic (21 days) diazepam treatment (0.5 mg/kg, intraperitoneally) on [3H]PK 11195 binding to the heart and cerebral cortex was studied. A significant increase in the density of peripheral benzodiazepine binding sites in the heart (18%) and cerebral cortex (19%) was observed. Following five days of withdrawal, the maximal binding capacity of these sites returned to normal control values. No alteration in the equilibrium dissociation constant was observed during the treatment and withdrawal period.