Throughout the United States and Canada the polychlorinated di-benzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) released in kraft pulp effluent have become a matter of intense public and regulatory concern. The central nervous system (CNS) is known to be one organ that is sensitive to environmental toxins. Although several studies have demonstrated biochemical changes in various regions of dioxin- and polychlorinated biphenyl (PCB)-exposed brains, no one has yet characterized the specific cellular nervous system changes due to dioxin exposure. To this end, we have begun to analyze the CNS of heron hatchlings contamined in the wild in ovo with a mixture of PCDDs and PCDFs. We used histologic and morphometric techniques to examine differences in the cell density of specific nuclei and the gross differences in the shape of different parts of the brain. Preliminary analyses indicated the following: a) the brains of heron hatchlings from contaminated colonies exhibited a gross morphometric intercerebral asymmetry, and such asymmetry was associated with the level of TCDD contamination in eggs; and b) there was an increased cell density and overall medial-to-lateral width in the pyriform cortex of the brains of hatchlings from a contaminated versus a relatively uncontaminated colony.