The desulfuration of thiopental to pentobarbital has previously been shown to be a relatively minor pathway of thiopental metabolism. In two cases, we observed significant conversion, resulting in blood pentobarbital concentrations up to 50 percent of total blood barbiturate (thiopental and pentobarbital) concentrations. Both patients received continuous infusions of thiopental and had present a condition (hypothermia) or drug (cimetidine) known to inhibit hepatic microsomal enzyme activity. It is suggested that inhibition of hepatic microsomal enzyme activity may prevent thiopental's metabolism to its major metabolite, a carboxylic acid analogue, and increase the amount of thiopental desulfurated to pentobarbital. Inhibition of hepatic microsomal metabolism also decreases the metabolism of pentobarbital. Until further elucidation of the causes of altered thiopental metabolism is available to identify patients more likely to have elevated concentrations of pentobarbital, monitoring of blood drug concentrations in patients receiving thiopental should include determination of both thiopental and pentobarbital concentrations.