Rodent post-implantation whole embryo culture (WEC) is a validated and widely used method in both mechanistic studies and as a screening test for developmental toxicants. Since metabolism is lacking in the WEC because of the developmental stage of the embryo, pro-teratogenic compounds are not detected. We investigated the inclusion of an in vitro metabolizing system as a pre-incubation step in the existing WEC. We started by testing cyclophosphamide, a known pro-teratogen. Without metabolic activation, cyclophosphamide is not teratogenic to rat embryos, as evidenced by a lack of effect on either embryonic growth or on morphological development. After pre-incubation with Aroclor-induced hepatic microsomes, cyclophosphamide became highly embryotoxic in the WEC. We tested five additional compounds to prevalidate this method: valpromide, 2-acetylaminofluorene, 2-methoxyethanol, retinol, and benzo[a]pyrene. Results revealed that not all of these five compounds underwent (complete) metabolic activation. This is probably due to the fact that microsomes do not contain the full spectrum of hepatic enzymes. Attempts have been made to replace microsomes by S9 liver fractions, which do contain microsomal enzymes as well as a series of cytoplasmic enzymes. However, S9 appeared to be extremely toxic in the WEC. Future experiments have to be performed to explore alternative ways of complete metabolic activation.