Male mice deficient in relaxin showed retarded growth and marked deficiencies in the reproductive tract within 1 month of age. At 3 months of age, male reproductive organ weight (including the testis, epididymis, prostate, and seminal vesicle) from relaxin null (RLX-/-) mice were significantly (p < 0.05) smaller than those of wild-type (RLX+/+) male mice. Histologic examination of RLX-/- mouse tissues demonstrated decreased sperm maturation (testis), increased collagen, and decreased epithelial proliferation in the prostate compared with tissues obtained from RLX+/+ animals. The degree of sperm maturation in the testes of sexually mature RLX-/- mice (3 months) resembled that of immature (1 month) RLX+/+ mice and correlated with a decrease in fertility in RLX-/- mice. The marked differences in the extracellular matrix of the testis and prostate in RLX-/- males also correlated with an increase in the rate of cell apoptosis. Relaxin and LGR7 (relaxin receptor) mRNA expression was demonstrated in the prostate gland and testis of the normal mouse. Data from this study demonstrate that relaxin is an important factor in the development and function of the male reproductive tract in mice and has an essential role in the growth of the prostate and maintenance of male fertility. Relaxin may mediate its effects on growth and development by serving as an antiapoptotic factor.