Summary. Adrenergic antagonists were administered to rats by intratesticular injection at the time of unilateral orchidectomy and 5 h before autopsy, 24 h after surgery. Injections of the β-receptor antagonist dl-propranolol (0·5 or 1·0 mg/injection) significantly inhibited the increase in the concentration of androgens in testicular vein plasma or interstitial fluid that occurred in unilaterally orchidectomized animals injected with vehicle. dl-Propranolol injections in animals with both testes did not reduce testicular or peripheral androgen concentrations or their increase after hCG administration. Injections of the less potent isomer (+)-propranolol or the α-receptor antagonist phentolamine did not inhibit the response to unilateral orchidectomy. It is concluded that the compensatory increase in androgen secretion induced by unilateral orchidectomy is, at least in part, the result of β-adrenergic stimulation of steroidogenesis.