Thirty-seven newly admitted schizophrenic patients were treated with an open and flexible dosage of chlorpromazine for 3 months after receiving a test dose. Levels of chlorpromazine, demethylated chlorpromazine and chlorpromazine sulfoxide 3 h after the test dose were measured. Twenty-three patients responded to long-term chlorpromazine treatment but 14 did not, a rate of 62.2%. A discriminant function analysis was performed using variables relating to the patients, backgrounds added to the ratios of plasma drug levels separately by sex to increase predictability over the level of previous studies. The obtained equations were applied to 23 newly admitted schizophrenic patients, with the prescription of chlorpromazine for designated responders and haloperidol for designated non-responders for 4 weeks. The patients in the latter study responded better than those of the former with chlorpromazine alone; 71.4 and 88.9% of chlorpromazine- and haloperidol-treated groups improved, respectively, for an overall rate of 78.3%. However, the chlorpromazine-treated group had a lower level of positive symptoms than the haloperidol-treated group before treatment and this and other differences between the groups should be further examined.