C57BL/6J mice were poisoned with methylmercury during pregnancy, and the location of heavily labeled neurons generated at embryonic day 16 was determined by tritiated thymidine autoradiography of the cerebral cortex of offspring at postnatal day 10. Camera lucida plotting of the distribution of radioactively labeled cortical neurons revealed statistically significant differences between control and methylmercury-treated groups. While control animals showed regular and tight packing of labeled neurons within the upper part of the cortical layer II, in methylmercury-treated animals such neurons were irregularly distributed throughout cortical layers II and III. Short-term intermittent and long-term low-dosage regimens of intoxication produced similar results. These findings support the hypothesis that prenatal methylmercury poisoning results in abnormal neuronal migration and anomalous cortical cytoarchitectonic patterning within the developing brain and provide a possible morphological basis for some of the neurobehavioral abnormalities that may be observed in association with sublethal prenatal intoxication in humans.