An oral assay 10-day procedure for bioassay of androgens is described using the immature castrated male albino rat and the end-points of seminal vesicles, ventral prostate, and levator ani tissues. No particular gain in precision could be achieved by extending the assay procedure beyond ten days. Using single daily doses and an aqueous suspending medium, the [lambda] value for the seminal vesicles was 0.210, for the ventral prostate it was 0.190, and 0.218 for the levator ani. The present experiments were carried out with fluoxymesterone.