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Administration of apomorphine hydrochloride (0.5 mg kg−1 s.c.) to adult male or female Wistar rats previously acclimatized to the test environment induced climbing behaviour in approximately 50% of animals examined. The proportion of animals climbing was related to age, being maximal at 8–9 weeks. Those animals showing an initial climbing response to apomorphine (0.5 mg kg−1 s.c), climbed when challenged with this dose of apomorphine on subsequent occasions. In ‘climbing’ animals the intensity of response was related to the dose of apomorphine administered; no dose‐response relationship was observed in ‘non‐climbing’ animals. No overall differences in the spontaneous motor behaviour of ‘climbing’ and ‘non‐climbing’ animals were apparent as assessed by measurement of spontaneous climbing behaviour, by holeboard activity, and by locomotor activity measured in either photocell cages or in a treadwheel. There was no overall difference in the ability of apomorphine to induce locomotor activity or stereotyped behaviour in ‘climbing’ and ‘non‐climbing’ animals. However, the administration of apomorphine induced rearing and treadwheel activity only in those animals classified as ‘climbers’. There was no difference between the number (Bmax) of specific [3H]‐spiperone binding sites or the dissociation constant (KD) in striatal or mesolimbic tissue preparations for ‘climbing’ and ‘non‐climbing’ rats. The ability of an animal to climb in response to apomorphine appears to be dependent on an ability to orient vertically, since this is a component of behaviour common to climbing, rearing, and treadwheel activity. The ability to climb does not appear to be related to differences in dopamine receptor numbers in brain or to the penetration of apomorphine into brain.

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