The effects of prenatal exposure of mice to the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C 4 mim]Cl) were studied because of the potential for human exposure as a result of water or soil contamination from industrial effluent or accidental spills. After exposure to the IL, fetal weight was significantly decreased at the two highest dosages (P ≤ 0.01). Malformations were also somewhat more numerous at the highest dosage, suggesting that [C 4 mim]Cl may be teratogenic, although the apparent increase was not statistically significant. Maternal toxicity was also present, as shown by a dose-dependent increase in maternal morbidity and mortality during the course of treatment. Therefore from these results, [C 4 mim]Cl appears to be developmentally toxic at maternally toxic dosages, although the mechanism is unknown.