Effects of Age, Sex, Castration and Interval of Time After Parturition upon the Ability of the Albino Rat to Build up Tolerance to and to Detoxify Pentobarbital Sodium*,†*From the Department of Physiology and Pharmacology, College of Pharmacy, University of Nebraska, Lincoln, Neb.†This project was aided by a grant from the Committee of Therapeutic Research, Council on Pharmacy and Chemistry, American Medical Association. We are indebted to Eli Lilly and Company for the Pentobarbital Sodium.