AbstractThe in vivo teratology screening procedure described previously [Chernoff N, Kavlock RJ: J Toxical Environ Health 10:541–550, 1982] was further evaluated using a total of 46 chemicals in 50 different treatment regimens. Pregnant CD‐1 mice were generally treated by oral gavage on days 8–12 of gestation at a dose level predicted from a preliminary range finding study to induce a slight degree of maternal toxicity. The effects on early postnatal growth and viability were compared to results generated from standard mouse teratology bioassays as reported in the literature (there were nine regimens for which no valid comparisons could be made). The procedure correctly categorized 25 of the 30 treatment regimens which were considered developmentally toxic in the mouse, as well as nine of 11 which were considered to be nondevelopmentally toxic in the mouse. Thus, based upon the criteria used in the present study, the assay correctly classified 83% of the chemicals tested as to their effect in a standard mouse bioassay. The five nonconcurring negative findings were likely due to a combination of pharmacokinetic differences between the studies, as well as to the cessation of dosing on day 12, while critical events of organogenesis are still occurring. The assay achieves the requirements for a teratology screening system, but improved predictability would result from the addition of a lower dose level and extension of the dosing period to include later stages of organoge