Experiments were conducted to evaluate the effects of alprazolam (1 mg/kg i.p.) on the development of autochthonous tumors induced by the Moloney sarcoma virus (MSV) in BALB/c female mice subjected to stress. Enhancement of MSV-induced tumor incidence and growth was observed, together with a delay in the usual prompt regression of the tumors, when programmed white sound anxiety-stress was administered immediately following MSV i.m. inoculation. However, a depressant effect on tumor size and incidence was observed when stress was administered before virus inoculation. Treatment with alprazolam was found to reverse partially the adverse effects of postinoculation stress, and also to inhibit the beneficial effects of the preinoculation administration of stress on tumor development. Pretreatment with Ro 15-1788 (10 mg/kg s.c.), a central benzodiazepine antagonist, resulted in a suppression on both effects of alprazolam in stressed mice.