• Pharmacokinetics of the phenothiazine, butaperazine, were studied in relationship to acute dystonic reactions. Dystonias appeared on falling drug concentrations, more than one half-life after plasma and red blood cell (RBC) peak butaperazine concentrations. Red blood cell butaperazine kinetics differentiated better than did plasma butaperazine levels those subjects in whom dystonias would develop from those in whom they did not. We conclude that RBC phenothiazine levels may more clearly reflect drug concentration at critical brain sites than do simple plasma drug levels. Furthermore, dystonic reactions may be the result of differential sensitivity of two or more receptor systems to receptor blockade by antischizophrenic agents.