Rat embryos undergoing organogenesis (day 10.5 of pregnancy) were exposed to 1.0 mM saccharin or 0.1, 0.3, or 1.0 mM-cyclohexylamine in an in vitro metabolic activating system, 1.0 mM-saccharin had few adverse effects on yolk-sac growth or embryo growth or differentiation. However, rat embryos cultured in vitro with cyclohexylamine and an adult rat hepatic microsomal activating system showed more growth retardation and dysmorphogenicity than embryos exposed to the same concentrations of cyclohexylamine alone. A concentration of 1.0 mM-cyclohexylamine alone reduced yolk-sac DNA content (by 51%), embryo DNA content (69%) and crown-rump length (26%). In the presence of a hepatic microsomal activating system this same dose of cyclohexylamine decreased yolk-sac DNA (60%), embryo DNA (84%) and crown-rump length (49%). The effects of the metabolic activating system and of cyclohexylamine were additive.