AMONG the causes of unexpected responses to coumarin anticoagulants are alterations in the rate of metabolism of these drugs. Several years ago certain barbiturates were shown to be capable of depressing the anticoagulant response to ethylbiscoumacetate and bishydroxycoumarin (Dicumarol)1,2in man. This depressed response correlates with an increased rate of metabolism of the anticoagulant drugs.3The present report suggests that chloral hydrate behaves in much the same manner as barbiturates. Since certain drugs increase the rate of metabolism of simultaneously administered bishydroxycoumarin, it is likely that discontinuance of the same drugs will result in a decrease in the rate of metabolism of bishydroxycoumarin. Such a decrease would result in an elevation of plasma levels of the anticoagulant which in turn would produce a prolongation of the prothrombin time. This mechanism appears to have precipitated a fatal hemorrhage in a patient treated with bishydroxycoumarin. Report of a Case A