In the absence of scheduled shocks cyclazocine (2.5 mg/kg) severely depressed avoidance responding. No behavioural excitation was observed. Behavioural excitation is observed after cyclazocine (2.5 mg/kg) injection, in the presence of scheduled shocks. The excitatory phase associated with the drug is dependent on the presence of the aversive stimulus. Thus, behaviour/drug interaction is dependent on environmental factors. The interaction of stress on psychotomimetic drug-action allows one to formulate a mechanism whereby environmental stress can play a crucial role in the genesis and underlying mechanism of schizophrenia.