Hepatic microsomal enzyme systems can be activated by pretreatment with phenobarbital, other barbiturates, ddt, and many other drugs. Contrarily, another compound such as skf 525-a, ethionine, and puromycin can inhibit the same enzymatic mechanisms. The sleep-time periods of albino rats produced by pentobarbital, thiopental, zoxazolamine, ci-581, diazepam, and gamma-hydroxybutyrate were significantly shortened by previous administration of phenobarbital twice daily for four days. However, the duration of action of the same anaesthetics was significantly prolonged when the animals were “pretreated” by the administration of skf 525-a, an inhibiter of liver microsomal enzymes.