Abstract: Gallanilide, N- (β-diethylaminoethyl) -N- (o-methylphenyl) -3', 4', 5', -trimethyloxycarboxylamide maleate (603), is a new synthetic compound with antiarrhythmic action. Antipyrine plasma half-life (t(1/2)) was used as an indicator by quantitative TLC scanning technique for studying the effects of gallanilide and phenobarbital on hepatic drug metabolizing enzymes in hybrid and inbred rats. I.p administration of gallanilide (8 mg/kg, twice a day) or phenobarbital (40 mg/kg, once daily) to both hybrid and inbred rats for three days, caused significant prolongation or shortening of antipyrine plasma t(1/2), respectively. The results showed that gallanilide probably inhibited hepatic drug metabolizing enzymes and that phenobarbital induced them, significantly. But it was found that there was no statistical difference between the two species of animals. The results suggest that hybrid rats could be used for primary investigation.