Twenty-one chronic schizophrenics were stabilized with chlorpromazine therapy at their therapeutic dosage for one month. Trihexyphenidyl hydrochloride or identical placebo was then added according to a double-blind, split crossover design. The duration of each half of the crossover was 15 days. Steady state blood samples were drawn three times weekly during the experimental period and the amount of chlorpromazine was determined. The results indicated there were no differences in the levels obtained between the trihexyphenidyl and the placebo phases. A two-hour postdrug blood sample was also drawn at the end of each phase and again, there were no differences between the two conditions. The importance of these results is discussed.