Purpose: The etiology of the worldwide incidence of hypospadias remains unexplained. Data from the Center for Disease Control suggest that maternal asthma exposure may be a risk factor. The development of the mouse and human urethra shows significant similarities, such as fusion of 2 epithelial edges, closure of a midline epithelial seam and subsequent cellular remodeling, to justify the use of the mouse as an experimental animal model. Prednisone may act as a weak androgen directly or alternatively lowering testosterone by hypothalamic suppression of gonadotropins. We describe the effects of prednisone on urethral formation in the mouse.