OBJECTIVE To investigate the effects of fetal nanoparticle exposure on reproductive function in male mice offspring. METHODS Animal study. METHODS Academic research laboratory. METHODS Forty pregnant ICR mice and 120 male offspring. METHODS Two hundred microg of 14-nm carbon nanoparticles was administered intratracheally on days 7 and 14 of gestation, and reproductive function of male offspring was determined at ages 5, 10, and 15 weeks after birth. METHODS Maternal and fetal growth, histologic changes in the testes, and daily sperm production (DSP). RESULTS Histologic examination showed partial vacuolation of seminiferous tubules. and cellular adhesion of seminiferous epithelia was reduced at all three ages. In addition, DSP was significantly decreased in fetal carbon nanoparticle-exposed mice. The DSP in the fetal carbon nanoparticle-exposed mice decreased by 47% at the age of 5 weeks, by 34% at the age of 10 weeks, and by 32% at the age of 15 weeks. On the other hand, nanoparticle administration had no marked effect on body weight, testicle weight, epididymis weight, or serum testosterone concentration. CONCLUSIONS These findings suggest that fetal nanoparticle exposure affects the reproductive function of male offspring. In the future, it would be necessary to clarify the onset mechanisms of nanoparticle-induced male reproductive disorders.