Growing concerns regarding the possibility that environmental contaminants could mimic estrogens and androgens and hence disrupt endocrine systems have led to the evaluation of test systems to detect such contaminants. One system, the weanling rat assay, has been suggested as a possible non-invasive replacement for the Hershberger castrated rat test for androgens and antiandrogens. The weanling assay, measuring the growth stimulation of sex accessory tissues (SAT) to monitor for androgenic activity and the inhibition of testosterone-stimulated SAT weight increases by antiandrogens, has been used to investigate dose responses to both androgens and antiandrogens. The current proposed investigation involves two studies using the weanling assay to test the response to a series of defined coded compounds. One study will monitor androgenic activity; the other will monitor antiandrogenic activity. Thus, the assay’s reproducibility and robustness will be tested in different laboratories, using different personnel, with a series of carefully controlled and defined coded compounds.