Exposure of pregnant CD-1 mice to methanol (MeOH) by inhalation on gestation days (gd) 6-15 results in dose-related increases in fetal cleft palate, exencephaly, and skeletal defects. Here, critical periods for the developmental toxicity of MeOH were assessed in pregnant CD-1 mice exposed to 10,000 ppm MeOH or filtered air for 7 hr/day on 2 consecutive days during gd 6-13, or to single day (7 hr) exposures to 10,000 ppm MeOH during gd 5-9. Mice received water but not food during exposure. Maternal blood MeOH was determined at times during, at the end of, and subsequent to a single 7 hr exposure on gd 7. On gd 17, remaining mice were weighed, killed, and gravid uteri removed. Live, dead, and resorbed fetuses were counted, and live fetuses were examined, weighed, and preserved in 70% ethanol. All fetuses were examined externally and for cleft palate, eviscerated, and stained with Alizarin red for skeletal examination. Pregnant mice lost an average of 0.3-2.9 g during 7 hr exposure to either filtered air or MeOH, but a MeOH treatment effect was evident only with 2-day exposure on gd 7-8. Peak maternal blood MeOH concentration (at the end of exposure) was approximately 4 mg/ml, and MeOH was cleared from maternal blood within 24 hr. Some fully resorbed litters were observed with 2-day MeOH exposures on gd 6-7 or 7-8, or 1-day exposure on gd 7. With 1-day MeOH exposure on gd 7, the number live was lower than with exposure on any other day. As previously reported, cleft palate, exencephaly, and skeletal defects were the fetal anomalies observed in this mouse strain. Cleft palate occurred with 2-day exposures on gd 6-7 through gd 11-12 (peak on gd 7-8), and with 1-day exposure on gd 5 through gd 9 (peak on gd 7). Exencephaly occurred with 2-day exposures on gd 6-7 through gd 8-9 (peak gd 6-7) or 1-day exposure on gd 5 through gd 8 (peak on gd 7). Skeletal elements malformed included the exoccipital (peak gd 6-7, gd 5), atlas (peak gd 6-7, gd 5,6), axis (peak gd 6-7, gd 7), cervical vertebra 7 with a rib (peak gd 6-7, gd 7), and lumbar vertebra 1 with a rib (peak gd 7-8, gd 7). An increased incidence of fetuses with 25 presacral vertebrae (normal = 26) was observed with methanol exposure on gd 5, whereas an increased incidence of fetuses with 27 presacral vertebrae was observed with MeOH exposure on gd 7. These results indicate that gastrulation and early organogenesis represent a period of increased embryonal sensitivity to methanol.