Cellular immediate-early genes are rapidly induced by a diverse range of agents and conditions. Since many cIE genes encode known or potential transcription factors, they are believed to couple extracellular stimuli to long-lasting alterations in cellular phenotype through the regulation of gene transcription. In addition, the localization of the products of cIE genes has been used as a method for determining the cellular sites of action of particular agents in the nervous system. However, the methods of analysis are tedious, and the results may be ambiguous because of cross-reaction of reagents with related proteins. To further the utility of this approach, a bacterial gene encoding beta-galactosidase (lac Z) has been fused, in frame, into the fourth exon of c-fos, and this fos-lac Z fusion gene has been introduced into the germ line of mice. We have analyzed the expression of beta-galactosidase (under the control of the c-fos promoter) in the developing and adult nervous systems of these transgenic mice. As far as can be determined, the constitutive and stimulated expression of the transgene accurately reflects the expression of cognate c-fos in both cultured cells and the intact animal. This study has also revealed novel sites of constitutive and induced expression of c-fos that were overlooked using conventional analysis. In particular, constitutive expression of c-fos is associated with cells that are entering terminal differentiation and are destined to die. In addition, induced expression of the transgene in adult brain mirrors the pattern of neurotoxicity elicited by kainic acid.(ABSTRACT TRUNCATED AT 250 WORDS)