The relationship of plasma prolactin to plasma or red blood cell butaperazine levels was investigated in chronic schizophrenic patients treated with clinical doses of butaperazine, both after a single acute dose of the drug and during regular, twice daily butaperazine administration. Although there was a significant curvilinear relationship between peak plasma butaperazine levels after an acute single oral dose of butaperazine and the maximum prolactin response that we measured, steady-state levels of plasma or red cell butaperazine and plasma prolactin were not related. We conclude that plasma prolactin cannot be used as a substitute for or even as a rough indicator of butaperazine blood levels in chronic schizophrenic patients being treated with clinical doses of butaperazine.