Abstract
Pretreatment of rats with phenobarbital for 4 days stimulates the activity of liver microsomal enzymes that metabolize 17α-ethynylestradiol-3-methyl ether (mestranol). This effect provides an explanation for the decreased uterotropic action of mestranol in rats pretreated with phenobarbital and may provide an explanation for unwanted pregnancies in women taking oral contraceptives in combination with phenobarbital or other enzyme-inducing drugs.