It is well established that clozapine is less likely than typical anti
psychotic drugs to cause clinically discernible extrapyramidal side-ef
fects. There is a paucity of data, however, on clozapine's motor effec
ts. In this report we compare normal controls to groups of chronic sch
izophrenic patients treated with either typical antipsychotic drugs or
with clozapine. Motor function was measured with a target-matching ta
sk, a test relying on submaximal sustained force control. Results indi
cated that patients on clozapine performed with significantly lower ac
curacy (greater variability) of force control. Even though the clozapi
ne patients were treatment resistant to typical antipsychotic drugs, a
nd many had a history of tardive dyskinesia, we postulate that the obs
erved deficit is likely due to clozapine treatment rather than to earl
ier treatments or other factors. The observed force control deficit ma
y be the result of an increase in myoclonus and a generally lower leve
l of overall motor activity.