OBJECTIVE Phthalate esters have been shown to induce testicular damage in both adult and immature rats; however, there have been, so far, few reports describing the age-specific effects of phthalate esters on testicular function. The aim of this study is therefore to investigate the age-specific effects of mono-n-butyl phthalate (MBP) on the testes in both prepubertal and mature adult rats. METHODS Both prepubertal male rats and adult mature male rats were fed special rat chow containing 1% MBP for 10 days. Control prepubertal and adult rats were fed standard commercial rat chow during the same period as the MBP-treated rats. After 10 days of feeding, all rats were killed, and the testes were removed. The weight of the testis was measured, and histological examination of the testis was performed. In addition, the frequency of an apoptotic cell appearance in the seminiferous tubules was determined in both MBP-treated and control groups. RESULTS In the prepubertal rats, the mean weight of the testes was significantly lower in the MBP-treated rats than in the control rats. A histological examination of the MBP-treated testes showed a decreased seminiferous tubular diameter and an inhibited maturation of germ cells in comparison to those of the control testes. Furthermore, apoptotic cells appeared more frequently in the MBP-treated testes than in the control testes. Although in adult mature rats, no significant difference was observed in either the testicular weight or the histological findings between the MBP-treated and control rats. CONCLUSIONS The oral administration of MBP to male rats was observed to produce more pronounced testicular damage in prepubertal rats than in adult mature rats. Immature testes may thus be more sensitive to MBP, which induces the germ cell apoptosis in seminiferous tubules and testicular atrophy in prepubertal young rats.