Utilizing vaginal cornification as a response for bioassay, a study was conducted to observe the variation in the median cornification dose (cED50) of estradiol, a hepatic-first-pass candidate, given either ip or sc in spayed rats, with or without enzyme induction by rifampin. Comparisons within and between the groups showed that after enzyme induction cED50 was increased fourfold and cED50 ip/cED50 sc ratio was doubled. The findings clearly demonstrate that this animal model faithfully reflects alterations in hepatic enzyme activity and could serve as an alternate for conventional hexobarbitone sleeping time test to study enzyme induction.