2,4-Pentadione (123-54-6) was evaluated for developmental effects in groups of 25 female Fischer rats administered the test substance by inhalation for 6 hours/day at concentrations of 0, 50, 200, or 400 ppm on days 6 through 15 of gestation. Animals were sacrificed on day 21 of gestation. Animals at 400 ppm showed signs of maternal toxicity (transient reduced body weight and reduced body weight gain during the exposure period). The 200 and 400 ppm groups showed reduced fetal body weights per litter. The incidences of external, visceral, skeletal and total malformations showed no differences from controls. The incidence of visceral variation and partial fetal atelectasis was increased at 400 ppm. Fetuses in this group also showed a consistent pattern of reduced skeletal ossification. The level of significance for all cases was p<0.05.