Environmental stimuli initiate metabolic events in neurons, one of the earlier being the activation of the gene, c-fos, and the expression of its protein product, Fos. We localized Fos after pharmacologically-induced seizure in both rats and chicks. Following 60 minutes of metrazole-induced seizure, animals were perfused intracardially with paraformaldehyde. Brain sections were processed using a polyclonal antibody to Fos and a Vectastain ABC kit, then mounted and photographed. In rats labeling was observed in the dentate gyrus and hippocampus, and in cingulate and pyriform cortex, results previously reported by others. The central grey and the medial and lateral pontine nuclei were also heavily labeled. In chicks, Fos immunoreactivity was much less pronounced than in the rat, but labeling was seen in the forebrain, central grey, and in both parvo- and magnocellular components of nucleus isthmi. We also investigated whether social isolation was capable of initiating similar cellular activity. Chicks were isolated for one-half, one, two, or four hours and then processed as above. Socially-housed chicks or those isolated but surrounded by mirrors were used as control. Thirty or sixty minutes of isolation resulted in labeling in forebrain, whereas 2 or 4 hours resulted in labeling of midbrain areas including central grey and n. isthmi.