Dysmorphogenesis elicited by microinjected acetaminophen analogs and metabolites in rat embryos cultured in vitro. Direct additions of acetaminophen (APAP), 3,5-dimethylacetaminophen, 3-hydroxyacetaminophen or 3-methoxyacetaminophen to the medium of cultured embryos each produced an increased incidence of morphologically similar, abnormally open anterior neuropores. Approximate concentrations required to produce an equal incidence were 0.5 mM, 1.0 mM, 0.1 mM and 0.75 mM, respectively. In contrast, 2.6-dimethylacetaminophen and N-acetyl-p-benzoquinoneimine failed to produce elevated incidences of abnormal neurulation unaccompanied by marked growth retardation. However, with intra-amniotic microinjections, 3-hydroxyacetaminophen and N-acetyl-p-benzoquinoneimine were roughly equipotent for eliciting abnormal neurulation, whereas 3-methoxyacetaminophen required greater than 30-fold higher concentrations. This suggests that N-acetyl-p-benzoquinoneimine does not readily transit the visceral yolk sac and would likely not be a major factor in APAP-elicited neural tube abnormalities unless generated in target tissues. The differential effects produced by two dimethylated (2.6 and 3.5) APAP analogs further suggest that sulfhydryl oxidation is associated more closely than sulfhydryl conjugation with the neurulation defect. Intra-amniotic microinjections of large quantities (3500 ng) of 7-hydroxy-2-acetylaminofluorene (7-OH-AAF) or APAP failed to produce the specific neurulation defect. Microinjections of 7-OH-AAF into the exocoelomic cavity effected the characteristic abnormal neurulation. Conversion by conceptal homogenates of 7-OH-AAF was roughly 7- to 8-fold more rapid than conversion of APAP to respective catechol metabolites, and specific activities in yolk sac tissues were greater than those in the embryo. Rates of conceptal conversion to the quinoneimine were approximately 2- to 3-fold lower than catechol generation. 