Mestranol and norethynodrel are synthetic steroids commonly used in oral contraceptive drug formulations. The efficacy of such preparations may be affected by other drugs which influence the hormonal activity of these agents.The present study utilizes the induction phase response of the rat uterus to a single dose of hormone as a method of evaluating possible interactions between mestranol, norethynodrel, and various other drugs.MateriaZs and Methods. Immature female rats (19 days old) were obtained from the Holtzman Colony, Madison, Wisconsin. The rats were caged individually and receivedPurina laboratory chow and water ad Zibiturn. Animal quarters were kept at a constant temperature (24') and on a controlled constant lighting schedule (12 hr dark, 12 hr light).Drugs were dissolved in saline with the exception of phenylbutazone and diphenylhydantoin, which were suspended in carboxymethylcellulose medium. Dru.gs were administered intraperitoneally twice daily for 3 days at the following doses: phenobarbital, 3 7 mgJkg; SKF 525A (Proadifen) 40 mg/kg; tolbutamide, 125 mg/kg ; chlorpromazine, 5 mg/kg; imipramine, 8 mg/kg: paraaminosalicylic acid, 3 mg/kg ; diphenylhydantoin, 12 5 mg/kg ; phenylbutazone, 60 mg/kg ; scopolamine hydrobromide, 0.5 mg/kg. The total volume per dose was 0.2 ml .Mestranol and norethynodrel were dissolved in corn oil and administered in a volume of 0.2 ml by oral intubation 18 hr following the last pretreatment dose.