Cellular changes attributable to copper were observed in the mechanoreceptors of the lateral line canals in the head of adult mummichog (Fundulus heteroclitus) and Atlantic silverside (Menidia menidid). The epithelium of these canals was also altered in F. heteroclitus. In both species, lesions were observed in the olfactory organs, which included the chemoreceptive sites. These manifestations of copper poisoning were evident at all studied concentrations (0.5, 1.0, and 5.0 mg/liter) of the metal. In M. menidia dilation of blood vessels was apparent, and in five cases hemorrhage had occurred in the brain and in periorbital connective tissues.Renal lesions in F. heteroclitus exposed to 1.0 and 5.0 mg/liter of copper were apparent; these changes could not be identified in M. menidia. Hepatic changes were not detectable by light microscopy in either species following external exposures to copper. High concentrations of copper administered intraperitoneally to F. heteroclitus did induce liver damage.Fry of F. heteroclitus were more sensitive to copper than were the adults or their zygotes. The emergence of larval forms from the zygote, the time required for emergence, and their survival was impaired by the metal. Lesions were not evident in developing sensory areas of the lateral line or the olfactory systems in these immature forms.