AbstractMale rats were treated daily with either intraperitoneal or oral doses (200 mg/kg) of a biflavonoid fraction of an extract fromGarcinia kolaseeds, kolaviron, either dissolved in DMSO or suspended in 0.9% NaCl. The duration of kolaviron treatment lasted 4 days (i.p.) or 7 days (oral). The control groups received 0.9% NaCl orally or DMSO alone (i.p.). The effect of kolaviron was tested on the duration of barbiturate‐induced sleep, and on the activities of liver microsomal drug metabolizing enzymes (p‐hydroxylation andN‐demethylation). Kolaviron was observed to: 1, significantly prolong the duration of sleep induced by either hexobarbital or pentobarbital; and 2, significantly depress the levels of drug metabolizing activity of the hepatic microsomal enzymes studied, when compared to the control groups. The effects of kolaviron pretreatment on the duration of sleep induced by barbiturates, are attributable to an inhibitory effect of kolaviron on the activity of microsomal drug metabolizing enzymes in the