In order to determine the potential developmental toxicity of ethylene glycol monopropyl ether (EGPE), pregnant rats were exposed to vapor concentrations of 100, 200, 300, or 400 ppm of the compound for 6 hours per day on days 6-15 of gestation. Maternal effects included a slight reduction in red blood cell count and increased mean corpuscular volume and mean corpuscular hemoglobin at the 200-, 300-, and 400-ppm concentrations. Reticulocyte counts and polychromasia of the red blood cells were increased at all exposure levels, while anisocytosis was increased at 300 and 400 ppm and macrocytosis was increased at 200, 300, and 400 ppm. Hematocrit, hemoglobin concentration, platelet, and total and differential white blood cell counts were comparable to those of the controls. Red urine was seen in the females after the first and second exposures to 200, 300, and 400 ppm of EGPE, but not after subsequent exposures. Absolute and relative spleen weights were increased by 200, 300, and 400 ppm EGPE. Histologic changes were seen in the maternal spleen, liver, and thymus, particularly after exposure to 300 and 400 ppm. Kidneys, bone marrow, and mesenteric lymph nodes were normal. Pregnancy rate, number of corpora lutea, implantation sites and viable fetuses per dam, the incidence of resorptions per litter, and the mean fetal body weights were comparable to those of the controls. Gross external, internal soft tissue, and skeletal examinations of the fetuses revealed that EGPE did not produce teratogenicity or significant embryo/fetotoxicity in the rat at vapor concentrations as high as 400 ppm. Variations in the ossification of certain skeletal elements and the incidence of 14th thoracolumbar rudimentary ribs were increased by exposure to 200, 300, and 400 ppm EGPE.