Apomorphine, d-amphetamine, chlorpromazine and dopamine, added to the aquarium water, were tested for their effects on the aggressive responses of isolated fish to mirrors and to models, compared with the responses of the same fish as non-treated controls tested at the start of, in the middle, and at the end of the experiment (Experiment 1). The effects of d-amphetamine administered by intracranial injection were also evaluated for responses to mirror presentations (Experiment 2). The three sets of controls in Experiment 1 indicate that the responses of the fish apparently changed over the 32-day experiment: they became more aggressive to mirrors whilst reacting less to models. Dopamine had no behavioural effect. Immersion in either apomorphine or d-amphetamine resulted in increased swimming activity, accompanied by apparent behavioural stereotypy. Immersion in apomorphine (Experiment 1) and intracranial injections of d-amphetamine (Experiment 2) also resulted in reduced aggression. Likewise, chlorpromazine also reduced some measures of overt aggression, whilst the appearance of other behaviours suggested that there is also an increased tendency to withdraw from the stimulus. It is concluded that the dopaminergic system plays an important role in the regulation of aggressive behaviour in this cichlid fish.