Thirty-four newly admitted schizophrenic patients were treated with a fixed dose of thiothixene (0.44 mg/kg) for 4 weeks. Thiothixene and its active metabolites were measured by a new radioreceptor assay. Improvement occurred over the entire range of recorded plasma levels, but the chances of substantial improvement appear greater above 40 neuroleptic units (n.u.). The data do not support the notion of a "therapeutic window", in that higher plasma levels were not associated with side effects or clinical deterioration (although at extreme plasma levels this must of course be so). In 11 nonresponders dosage could not be increased because of side effects. If a non-responder with troublesome side effects has a low plasma level (less than 40 n.u.), it would seem prudent to switch to another antipsychotic drug.