In a test of conditioned punishment, saline-treated controls showed a moderate bias in responding away from a lever producing a response-contingent auditory conditioned stimulus (CS) that had been paired with mild footshock during training and towards a lever producing a neutral auditory CS. Systemic treatment with the indirect dopamine (DA) agonist amphetamine (0.25-1.0 mg/kg) produced a dose-dependent increase in the punishing effect of the aversive CS, whilst responding on the neutral CS lever was unchanged. Treatment with the dopamine-receptor antagonist alpha-flupenthixol (0.125, 0.25 mg/kg) decreased the efficacy of the punishing CS, but again left responding on the neutral lever unchanged. The benzodiazepine midazolam (0.1, 0.3 mg/kg) had a similar effect to alpha-flupenthixol, but treated animals showed a preference for the aversive CS. Parallel results were observed with amphetamine (0.25 mg/kg) and alpha-flupenthixol (0.125, 0.25 mg/kg) in a matched test of positive conditioned reinforcement, with amphetamine enhancing, and alpha-flupenthixol reducing, the efficacy of the CS paired with food. Midazolam treatment (0.1-1.0 mg/kg) had no effect on the reinforcing impact of an appetitive CS. Thus dopaminergic agents modulate the behavioural impact of both appetitively and aversively motivated conditioned stimuli on instrumental performance, whilst the benzodiazepine midazolam has a selective impact on aversively-motivated stimuli that is qualitatively distinct from that of the dopaminergic antagonist alpha-flupenthixol.