The developmental toxicity of tetramethylurea (TMU) was assessed in rats by inhalation exposure of the test material over days 6-20 of gestation. Groups of 25 mated female Crl:CD BR rats were exposed whole-body for 6 hours/day to concentrations of either 0, 2, 20 or 100 ppm TMU. The dams were euthanized on day 21 and the offspring were weighted, sexed, and examined for external, visceral, and skeletal alterations. Maternal toxicity was demonstrated at both 20 and 100 ppm. Maternal body weights, weight changes, and food consumption were statistically significantly reduced at these concentrations; effects were more pronounced at 100 ppm. There was evidence of developmental toxicity only at 100 ppm. The only finding was a decrease in mean fetal weight. No fetal malformations or variations occurred in fetuses derived from rats exposed to all 3 test concentrations (up to 100 ppm). The maternal no-observed-effect-level (NOEL) was 2 ppm, the fetal NOEL was 20 ppm. Thus, TMU was not considered to be uniquely toxic to the rat conceptus.