We have investigated the feasibility of administration of testosterone undecanoate (TU)-loaded injectable in situ-forming implant (ISFI) for contraception in adult male Sprague-Dawley rats. Male rats were treated with vehicle, TU-loaded ISFIs (540, 270 and 135 mg TU kg-1 ) or TU injections (45 mg TU kg-1 every 30 days) for 120 days. Fertility tests served for determining infertility or restoration of fertility in treated rats. Serum testosterone concentration, epididymal sperm count, motility, morphology, and histology of the testis were monitored. The TU-loaded ISFIs increased serum testosterone levels in rats steadily without fluctuation over 3 months. One month after TU administration, the epididymal sperm count decreased significantly in all experimental groups. After 3 months, the animals treated with 270 and 135 mg kg-1 TU-loaded ISFIs were 100% infertile, and no implantation sites were produced in the mated females. However, some of males treated with 540 mg kg-1 ISFI or TU injections were still fertile but numbers of implantation sites were also significantly lower than control values. TU-loaded ISFI at an appropriate dose has potential as a long-acting male contraceptive drug that suppresses spermatogenesis consistently over a period of 3 months.