Abstract Pretreatment by oral administration of trichloroethyl phosphate produced no indication of enzyme induction in rats as revealed by the duration of the effect of the test material or of zoxazolamine. The half-life of bishydroxycoumarin in dogs was reduced by pretreatment with chloral hydrate, but not by trichloroethyl phosphate. Plasma bishydroxycoumarin concentrations and prothrombin times were more affected by concurrent administration of the anticoagulant and chloral hydrate than by the anticoagulant and trichloroethyl phosphate. Chloral hydrate produced significantly higher plasma concentrations of trichloroacetic acid than did trichloroethyl phosphate in the dog. Possible differences in metabolism of trichloroethyl phosphate with respect to species, man and dog, are pointed out.