Weanling rats were treated with diazepam, apomorphine, and haloperidol to study the influence of the dopamine (DA) system on the audiogenic immobility reaction and open field locomotory behavior. Treatment with diazepam (0.025, 0.05, and 0.1 mg/kg) caused a dose-dependent shortening of the duration of the immobility response. Treatment with apomorphine (0.125, 0.25, and 0.50 mg/kg) shortened both the immobility reaction and the latency to leave the spot where the animal was first placed in the open field (latency for first crossing). Locomotor activity increased in a dose-dependent fashion. Both grooming and rearing showed biphasic dose response curves, with a maximum occurring at the 0.125 mg/dose for grooming and at the 0.25 mg/dose for rearing. Haloperidol (0.06 mg/kg) exerted opposite effects to those of apomorphine, but also produced increased running during the auditory stimulation (flight distance). Using the immobility reaction as an expression of fear, we concluded that activation of the DA system decreases while inhibition of the DA system increases fear. It was hypothesized that the DA system exerts an inhibitory function in the expression of fear.