Morphologic lesions have received only limited attention as in vitro endpoints of toxicity. In the present work, "tissue" and cell morphology of control and toxicant-treated primary dissociated cerebrocortical cell cultures from fetal mice were examined using phase-contrast and bright-field microscopy. In untreated control cultures, a reproducible sequence of developmental events included cellular reaggregation, intercolony bridging with cell migration, and neuronal apoptosis, with maturation yielding confluent monolayers containing both neurons and glia. Because even mature cultures had regions of varying differentiation, an understanding of the normal developmental sequence was essential when assessing toxicant-treated cultures for damage. Chemicals induced neuronotoxic, gliotoxic, and cytotoxic (i.e., nonspecific) patterns of morphologic damage in growing (< 6 day old) or mature (6-15 day old) cultures in both a concentration-dependent and cell type-specific manner. In addition, exposure to some toxicants consistently reduced the staining intensity for glial fibrillary acidic protein in the astrocyte carpet prior to the appearance of structural damage. These data indicate that histopathologic endpoints, including methods for neural-specific markers, represent potentially valuable criteria for in vitro assessments of neurotoxicity.