1. Male rats were injected with either saline, diazepam, MK-801, or diazepam plus MK-801. 2. In previous work with phencyclidine (PCP), diazepam significantly reduced the increase in homovanillic acid (HVA) in olfactory tubercle and prefrontal cortex. 3. Diazepam also lowered the HVA increase following MK-801 in caudate, olfactory tubercle, and prefrontal cortex. 4. Benzodiazepine receptors may modify dopaminergic function at PCP receptors that affect dopamine neurotransmission.