Single i.p. doses of O,O,O-triethyl phosphorothioate [OOO-Et(S)], one of the suicide substrates for cytochrome P-450, caused a rapid increase of NADPH-cytochrome c reductase activity in rat liver microsomes. The increase was dose dependent but did not coincide with the recovery from the inhibition of drug-metabolizing activities. There was no change of Km value of the reductase in the induced state. The co-administration of cycloheximide repressed the stimulatory effect of OOO-Et(S), suggesting that a de novo synthesis of enzyme protein may be responsible for the increase in activity. Of four homologous tri-n-alkyl esters tested, the triethyl compound was the most effective at 24 and 48 hr after administration. Triethyl phosphate, the oxygen analog of OOO-Et(S), also caused an increase of the reductase activity, but carbon disulfide had no influence on this activity. Although O,O,S-triethyl phosphorodithioate [OOS-Et(S)] and its n-alkyl homologs also caused the inhibition of drug-metabolizing activities and the increase of the reductase activity, the recovery and the stimulation of enzyme activity were different from that of O,O,O-tri-n-aklyl phosphorothioates.