AbstractActivation of c‐fos, a member of the class of immediate early genes that act as transcription factors, may be one of the initial molecular mechanisms underlying plastic changes in gene expression in response to drugs of abuse. By combining c‐fos (radioactive) in situ hybridization histochemistry with nonradioactive in situ hybridization histochemistry for mRNAs encoding other striatal markers [preprotachykinin (substance P), proenkephalin, and D1 and D2 receptors], we have identified the cellular phenotype of striatal neurons activated by acute administration of cocaine to P8, P15, P28, and adult rats. At each age examined, substance P+, enkephalin−‐ striatal neurons were the predominant class of cells in which cocaine induced c‐fos gene expression. In addition, the topography of cellular activation at each age examined was distinct and reflected the topography of distribution of cells expressing high levels of substance P mRNA. We conclude that there is a marked specificity of cellular activation in striatum following acute cocaine administration restricted predominantly to subsets of substance P‐expressing cells, with age‐specific patterns in their topographic distribution. © 1995 W