Mouse embryos were exposed to cadmium (5.35 micrograms/ml) for either 0.5h or 1h and then transferred to fresh serum for the duration of the experiment. Embryos exposed for 0.5h developed numerous malformations that were primarily localized to the craniofacial region. Embryos exposed for 1h to the same level of cadmium all failed to thrive. In a separate series of experiments zinc (10 micrograms/ml) was added 15 minutes prior to the addition of cadmium. Embryos treated with cadmium for 30 min. after the addition of zinc, resulted in 15 of 16 embryos developing normally. The second group of embryos treated for 1h after preincubation with zinc resulted in 93% of the embryos surviving after 48h, albeit malformed. These results suggest that: 1) Exposure time to a teratogen is a critical parameter to consider when designing in vitro experiments 2) zinc is effective in protecting embryos against cadmium induced teratogenesis and 3) the whole embryo culture system is effective in demonstrating biological interactions between agents.