A single injection of fluphenazine decanoate (FD) antagonized effects of acute d-amphetamine (2.5 mg/kg) for a variable period of time (4 to 28 days), depending upon the dosage of the neurolcptic used (2.5 or 5.0 mg/kg) and the behavioural parameter(s) monitored. Locomotion and rearing were antagonized for a longer duration than was sniffing. Normal locomotor response to amphetamine was attained 12 and 28 days following the administration of 2.5 and 5.0 mg/kg FD, respectively. However, the 2.5 mg/kg FD group displayed significantly more locomotor activity on days 20 and 24 post-treatment. A similar supersensitive response was not demonstrable with the higher dose of FD (5.0 mg/kg), or with the other behavioural measures. The prompt and pronounced elevation of serum prolactin in response to the neuroleptic returned to within the normal range by days 4 and 14, following administration of 2.5 and 5.0 mg/kg FD, respectively. These results indicate that the behavioural paradigm is more sensitive in monitoring the effects of FD and could serve as a useful model in investigating the doseand timerelated effects of other long-acting neuroleptics.