Abstract Pairs of osmotic minipumps containing 400 mg/ml (6.15 m) sodium azide in distilled water were subcutaneously implanted in timed pregnancy Syrian golden hamsters. The total delivered dose was calculated as 6 × 10 −2 mmol kg −1 hr −1 at the maximal pumping rate. Most dams exhibited obvious signs of toxicity during the period of pump implantation which was Days 7 through 9 of gestation. After removal of the pumps the dams were euthanized on Day 13 of gestation, and the uteri were removed for counting of the number of living, malformed, and resorbed fetuses. This dose rate resulted in a significantly increased incidence of resorptions of embryos over that in a control group implanted with pumps delivering only distilled water. The incidence of gross malformations exclusively in the form of encephaloceles was not different between control and azide-infused groups. The extent of nitrosylation of circulating hemoglobin was followed with time and found to involve only about 0.1% of the total blood pigment. Thus, this commercially important and widely distributed chemical with high acute toxicity is not considered to be teratogenic in hamsters, and it produces embryotoxicity only at dose rates that result in toxic signs in the dams.