This study was conducted to follow up the effects of fetal exposure to diesel exhaust on testicular cell numbers and daily sperm production in adulthood. Thirty-six pregnant rats were divided into five groups: groups exposed to total diesel-engine exhaust containing 1.71 mg/m3 particulate matter and 0.80 ppm nitrogen dioxide (high dose) or 0.17 mg/m3 particulate matter and 0.10 ppm nitrogen dioxide (low dose); groups exposed to filtered exhaust without particles containing 0.80 (high dose) or 0.10 (low dose) ppm nitrogen dioxide; and a group exposed to clean air. Exhaust exposure was performed from gestational day 7 to delivery. The numbers of daily produced sperm, spermatids and Sertoli cells in the diesel-exhaust-exposed groups were significantly lower than those in the control group on day 96 after birth. The ratio of spermatids/Sertoli cells and the follicle-stimulating hormone levels in the exposed groups were significantly higher. The present study provides evidence for the first time that mature rats exposed to diesel exhaust during fetus show a decrease in the daily production of sperm due to an insufficient number of Sertoli cells. As both the exhaust-exposed groups showed almost the same reactions toward the inhalation, the gaseous phase must have included the responsible toxicants.