1. The metabolism of nefiracetam, a novel cognition-enhancer, by rat liver microsomes has been studied. 2. Formation of 5-hydroxy (5-OH-NEF) and hydroxymethyl (HM-NEF) derivatives was the principal pathway of NEF oxidation in male rats, and followed Michaelis-Menten kinetics with Km values of 2.9 and 3.3 mM, and Vmax values of 7.8 and 4.5 nmol/min/mg protein, respectively. 3. Enzymes catalysing the formation of these two major metabolites were examined. 5-OH-NEF formation was inhibited by antibody to rat CYP3A2 by 60%, and antibodies to CYP2B1, CYP2C11 and CYP2E1 also showed 15-25% inhibition of the formation of 5-OH-NEF. The formation of HM-NEF was inhibited by antibodies to CYP2C11 and CYP2B1 by 80% and 35%, respectively. These findings indicate that CYP3A plays a major role in the formation of 5-OH-NEF, and CYP2B, CYP2C11 and CYP2E1 are also involved to some extent in the 5-hydroxylation, and that CYP2C11 is mainly responsible for HM-NEF formation, and CYP2B is also involved in that catalysis in male rats. The results from the studies of the effects of various chemical inducers, of selective substrates or inhibitors of P450s on the formation of these metabolites also supported these latter findings. 4. NEF metabolism in microsomes prepared from B-lymphoblastoid cells expressing human cytochrome P450s showed that 5-OH-NEF formation by CYP3A4 is the principal metabolic pathway in humans.