The Hershberger Bioassay is an in vivo short–term screening test. It evaluates the ability of a chemical to elicit biological activities consistent with androgen agonists, antagonists or 5 á-reductase inhibitors. The current bioassay is based on the changes in weight of five androgen-dependent tissues in the castrate-peripubertal male rat: the ventral prostate, seminal vesicle (plus fluids and coagulating glands), levator ani-bulbocavernosus muscle, paired Cowper’s glands and the glans penis. In order to establish whether a test substance can have androgenic or antiandrogenic action, two – respectively three - dose groups of the test substance, plus positive and vehicle (negative) controls are normally sufficient. The test substance is administered by gavage or subcutaneous injection daily for 10 consecutive days. To test for antiandrogens, the test substance is administered together with a reference androgen agonist. Each treated and control group should include a minimum of 6 animals. The animals are necropsied approximately 24 hours after the last administration of the test substance. The tissues are excised and their fresh weights determined. A statistically significant increase (androgenic) or decrease (antiandrogenic) in the weights of two of the five tissues indicates a positive response in this assay.