AbstractProcarbazine (PCZ) is an antineoplastic agent useful in the treatment of Hodgkin's disease, brain tumors, and chronic leukemia. PCZ is dysmorphogenic to developing embryos exposed in vivo or cultured in the serum of PCZ‐treated rats. However, embryos directly cultured with PCZ (up to 400 μg/ml) or PCZ plus S‐9 liver fractions are unaffected. Since intact liver cells provide several advantages over hepatic subcellular fractions for in vitro bioactivation, we exposed rat embryos to PCZ in an embryo/hepatocyte co‐culture system. Gestation day (GD) 9.5 rat embryos exposed to 0, 200, 300, or 400 μg PCZ/ml in the presence of untreated or phenobarbital induced male rat hepatocytes failed to display toxicity. However, in a companion study GD 9.5 rat embryos cultured in the serum from PCZ‐treated rats exhibited developmental deficiencies. Studies have shown that the formation of toxic metabolites can result from glutathione (GSH) conjugation of toxicants in the liver. Therefore, in a second set of experiments, rat embryos were cultured in serum from rats pretreated with two GSH depleters (phorone and buthionine sulfoximine) and subsequently dosed with PCZ. Effects on development were enhanced when embryos were cultured in the serum from PCZ‐treated/GSH depleted rats. These data indicate that PCZ requiresin vivoactivation to be dysmorphogenic and further suggest that the metabolite(s) responsible for procarbazine embryotoxicity are formed readily under conditions of low GSH levels. This argues against a glutathione conjugate as the ultimate toxicant. © 1995 Wiley‐Liss, Inc.This article is a US Government work and, as such, is in the public domain in the United Sta