AbstractMale Wistar rats treated with apomorphine (1.0 mg/kg) twice daily for several days exhibited two distinct behavioural responses characterized by stereotyped sniffing or ritualized fighting. The fighting response first appeared on the third day of drug administration and became more severe with repeated treatment. It differed from previously reported apomorphine‐induced aggression [McKenzie, 1971] in that dominant and subordinate rats did not emerge between pairs of fighting rats. Mild defensive fighting occurred in other animals after seven days of drug treatment. Marked differences were observed between aggressive and nonaggressive rats both during the acute effects of the drug and several hours after these effects had worn off. These differences included disturbance in the “open field” behaviour and the reaction to handling. Once a specific behaviour was established in an individual rat, it was not possible to induce a different reaction to apomorphine.It was shown that, in a group of rats similar in age, sex, strain, and body weight, chronic treatment with a low dose of apomorphine induced distinct and reproducible behavioural patterns. The standard rating scale for apomorphine‐induced stereotyped behaviour was shown to be inadequate for the study of such diverse beh