AbstractAcute exposures to sodium arsenite (50 μM) were embryotoxic in day 10 rat embryos exposed in vitro. Sodium arsenite–induced embryotoxicity was characterized by decreased growth (crown‐rump length, somite number, and embryo protein content) and abnormal development (hypoplastic prosencephalon, abnormal somites, and abnormal flexion of the tail). At embryotoxic exposures, sodium arsenite also induced the synthesis of three heat shock proteins (hsps), one of which is recognized by a monoclonal antibody specific for the heat‐inducible hsp 72. In addition, sodium arsenite induced the accumulation of heat‐inducible hsp 70 mRNA. Although the abnormal morphologies induced by sodium arsenite and hyperthermia appear to be different, the stress response as measured by the synthesis of hsps, the accumulation of hsp 72 protein, and the accumulation of hsp 70 mRNA is similar in embryos exposed to these two embryotoxic agents. Thus, sodium arsenite and hyperthermia both induce a stress response; however, the relationship between the induction of a stress response and the subsequent abnormal development that ensues is unclear. © 1992 Wiley