The effects of formate exposure on mammalian embryo development were investigated using the rat whole embryo culture system as a model. Day 9.5 (presomite) rat embryos were explanted and cultured for 48 hr in rotating bottles containing rat serum with 0, 0.2, 0.4, 0.8, 1.2 or 1.6 mg sodium formate/ml culture medium at pH 8.13, 7.75, 7.00, 6.50 or 6.00 to determine whether the pH of the culture medium affects the in vitro developmental toxicity of formate. Exposure to 1.6 mg formate/ml affected protein concentration, somite number (SN), head length (HL), developmental score (DS), crown-rump length (CRL) and yolk-sac diameter of embryos at all pH levels. Formate became more toxic with decreasing pH of the culture media. There was an apparent pH-dependent increase in embryolethality at 1.6 mg formate/ml and 100% lethality at pH 6.00. The 1.2-mg/ml formate concentration affected DS, CR, HL and protein content at pH levels of 7.00 and lower. At pH 6.5 embryos, that were not exposed to formate were not significantly different from the other control groups except in reduced CR but at this pH, all exposure levels of formate resulted in microcephaly and reduction in embryonic protein as well as reduced CR.