Methyl Ethyl Ketone (CAS No. 78-93-3) was evaluated for teratogenicity in groups of 25 (control group of 35) pregnant Sprague-Dawley rats exposed to the test substance by inhalation at concentrations of 0, 400, 1000, or 3000 ppm on 7 hours/day on days 6-15 of gestation. Rats exposed to 3000 ppm showed body weight gain on days 10-15 of gestation and a significant decrease in body weight on day 16. Water consumption was significantly increased on days 10 -15 in rats exposed to 3000 ppm. Rats were sacrificed on day 21 of gestation. The incidence of pregnancy, number of live fetuses per litter and resorptions were not affected at any of the exposure concentrations. The incidence of external, soft tissue alterations or major malformations was not significantly different from controls in any of the treatment groups. Skeletal alterations including an increased incidence of delayed ossification of intraparietal bones of the skull, increased incidence of extra lumbar ribs and delayed ossification of cervical centra were observed at 3000 ppm. There was not an increased incidence of major malformations. Minor skeletal alterations were seen as evidence of a slight fetotoxic effect. The authors concluded that the substance was not embryotoxic or teratogenic at levels up to 3000 ppm.