Navarro et al (2001) have reported that neonatal exposure of rat to the organophosphate compound chlorpyrifos (CPF) resulted in long-term deficits in T lymphocyte mitogenic response, although the mechanism has been unclear. In this study, pregnant BALB/c mice were exposed to 0, 2.8, 14, 70 ppm CPF via diet from gestational day 10 to postnatal day (PND) 21, and subpopulational changes in T lymphocytes of offspring were analyzed at PND21. The irreversibility of the effects was also investigated at PND77 after ceasing exposure by weaning at PND21. Serum cholinesterase activity was significantly reduced after exposure to CPF at PND21. An increase in the proportion of CD4 positive splenocytes was observed after exposure to CPF, which remained until PND77. We found that regulatory T cells were the only one CD4 positive subset which increased in the spleen of CPF-exposed mice at PND77.