Pigeons that repeatedly experienced the effect of apomorphine in the same environment showed an augmented behavioural response to the same drug dose as compared with controls that experienced the effect of the drug dose in differing environments. Sensitization, an increase in the behavioural response that is observed in pigeons when the same dose of apomorphine is repeatedly administered, may thus be mainly due to a conditioning of the drug response to incidental environmental cues. Apomorphine injections also induced place preferences. Pigeons that had experienced a particular environment under the influence of apomorphine subsequently favoured that environment to one they had experienced while under saline. This suggests that apomorphine administration has reinforcing properties for birds, much as it has for mammals.