A urea-selective urinary concentrating defect was found in transgenic mice deficient in UT-B. To determine the role of facilitated urea transport in extrarenal organs expressing UT-B, we studied the kinetics of [¹⁴C]urea distribution in UT-B null mice vs. wildtype mice. After renal blood flow was disrupted, [¹⁴C]urea distribution was selectively reduced in testis in UT-B null mice. In basal conditions, total testis urea content was 335.4 ± 43.8 mg in UT-B null mice vs. 196.3 ± 18.2 mg in wildtype mice (p < 0.01). Testis weight in UT-B null mice (6.6 ± 0.8 mg/g body weight) was significantly greater than in wildtype mice (4.2 ± 0.8 mg/g). Elongated spermatids were observed earlier in UT-B null mice compared to wildtype mice on day 24 vs day 32, respectively. The first breeding ages in UT-B knockout males (48 ± 3 days) were also significantly earlier than that in wildtype males (56 ± 2 days). In competing mating tests with wildtype males and UT-B null males, all pups carried UT-B targeted genes, which indicates that all pups were produced from breeding of UT-B null males. Studies of the expression of follicle stimulating hormone receptor (FSHR) and androgen binding protein (ABP) indicate that development of Sertoli cells was also earlier in UT-B null mice than that in wildtype mice. These results suggest that UT-B plays an important role in eliminating urea produced by Sertoli cells and that UT-B deletion causes both urea accumulation in the testis, and early maturation of the male reproductive system.