The secondary metabolite of dimethoxyethyl phthalate (DMEP), methoxyacetic acid (MAA), but neither the diester nor either of its primary metabolites, monomethoxyethyl phthalate (MMEP) and 2-methoxyethanol (ME), interferes with normal growth and development of organogenesis phase rat embryos in culture. These in vitro observations suggest that the teratogenicity of DMEP in vivo is due to enzymic cleavage of the diester to ME, followed by oxidation of the latter to MAA in the maternal compartment.