Dual action hypotheses have suggested that antipsychotic compounds may show a stimulating effect at low doses and an antipsychotic effect at higher doses. The clinical literature of thiothixene in adult schizophrenic patients was reviewed in an attempt to correlate optimal dose with chosen indices of the dual action hypothesis. Activation and side effects of CNS stimulation correlated highly with each other and with dosage; a significant negative regression line on optimal dose was seen. Overall efficacy did not correlate with dosage or with activation-stimulation, but showed the presence of an antipsychotic component at least equal to standard antipsychotics. The method employed seems suitable to test the hypothesis in other antipsychotic compounds.