Mono-n-butyl phthalate (MBP) was administered to pregnant rats from the 15th to the 17th gestational day to investigate the dose-dependent effect of phthalate ester on testicular descent in both pre- and postnatal rats. Thirty pregnant rats (280-330 g) were separated into five groups and administered 1.0 g/kg/day MBP in group I, 0.5 g/kg/day in group II, 0.25 g/kg/day in group III, 0.125 g/kg/day in group IV from the day 15 to 17 of gestation. The rats in group V were only administered solvent and acted as controls. The position of the testis was evaluated in both pre- and postnatal rats. This was significantly higher in 20-day old fetuses in groups I and II than in groups III, IV or V. After birth, the incidence of undescended testis was also higher in groups I and II compared with those of groups III, IV or V. The undescended testes were located in the abdominal cavity in groups I and II, whereas they were located at the inguinal region in group III. The lowest dose that induced an adverse effect on testicular descent was 0.25 g/kg/day. In conclusion, maternal MBP inhibited testicular descent in prenatal rats and thus dose-dependently induced undescended testes postnatally. High-doses of MBP are thus considered to inhibit the transabdominal descent of the testis, probably due to its estrogenic activity, whereas low-doses of MBP may act as an anti-androgen and thereby inhibit inguinoscrotal testicular descent in postnatal rats.