Plasma concentrations of chlorpromazine and its sulfoxide were measured after single intramuscular, single oral, and multiple oral doses of chlorpromazine. In four out of nine patients, intramuscular doses gave stable chlorpromazine plasma levels which were maintained for 12 hr or longer. Chlorpromazine sulfoxide was found in plasma from all patients after oral doses but not after intramuscular doses, which indicates that sulfoxidation took place presystemically. The biologic availability of single oral doses relative to single intramuscular doses ranged from 10 to 69%, and was on average 32%. After 33 days of oral dosing, the plasma levels of chlorpromazine were on average 37% lower than might have been predicted from the single oral dose curves. This was not due to decreased biologic t 1/2 of chlorpromazine, but apparently to a decrease in the biologic availability by oral administration, possibly by increased presystemic metabolism.