Trichlorfon (TCF) is a widely used broad-spectrum agricultural organophosphate (OP) pesticide. Few studies have evaluated the effects of TCF on reproductive toxicity after low-level exposure, especially after long-term exposure. This study assessed the direct effects of TCF on estrous cycle, oocyte maturation in female mice, and developmental outcome in near-term fetuses after 30 consecutive days of maternal exposure to 2, 10, or 50 mg/kg body weight/d TCF via drinking water. Both male and female fetuses in the 50 mg/kg/d TCF-treated group had significantly reduced body weights; but this did not occur in the 2 mg/kg/d and 10 mg/kg/d TCF-treated groups. No difference in oocyte maturation, including the percentages of germinal vesicle breakdown (GVBD) and first polar body (PB1) extrusion, or in estrous cycle was found between the control and TCF-treated groups. No increased incidence of fetal external malformations was observed in the TCF-treated groups. Significant decreases in maternal liver weights occurred in the 10 and 50 mg/kg/d TCF-treated groups in a dose-dependent manner. No significant changes were found in the weight of organs such as the ovaries, thymus, kidneys, spleen, lungs, heart or brain. The lack of effects of 2 mg/kg/d and 10 mg/kg/d TCF on any in vivo reproductive and developmental endpoints examined suggest that no TCF reproductive toxicity occurs at exposures less than 10 mg/kg/d.