Immediate early genes are rapidly and transiently expressed within neurons following stimulation. While it is likely immediate early gene products alter gene expression in neurons, the products of the Immediate early genes also serve as excellent markers for Identifying activated neurons. The immunocytochemical localization of immediate early gene products enables the analysis of changes In activation of individual neurons in the brain in response to gonadal steroids or other stimuli. The technique presented in this article outlines the strategies for using immediate early gene proteins in the fos family as markers for neuronal activity and focuses principally on the examination of one chemically identified neuron population, the luteinizing hormone-releasing hormone system. Included are comparisons of the staining of brain tissue for c-fos and related antigens following use of different tissue fixatives. Because some of the c-fos antibodies also recognize other proteins in the fos family, we have presented data comparing staining patterns obtained with a number of available antisera. We also describe some of the approaches we use to analyze c-fos expression.