Expression of c-Fos, or other immediate early gene products, by individual neurons can be used as a marker of cell activation, making staining of these proteins an extremely useful technique for functional anatomical mapping of neuroendocrine systems. Because these proteins are located in the nucleus, identification of the phenotype of the activated neuron using substances located within the cytoplasm can be accomplished with standard double-labeling immunocytochemical techniques. Although it is clear that neurons have the capacity to express a number of immediate early gene products, what remains to be established is whether there is a different pattern of expression following various stimuli. In our studies, we focus primarily on expression of one immediate early gene product, the c-Fos protein. We also include some experiments using expression of other members of the Fos family and Jun proteins as markers for neuronal activation. Our studies describe uses of c-Fos expression in both parvocellular and magnocellular hypothalamic systems to address the following issues: (a) identification of neuroendocrine cells activated by specific treatments and conditions, (b) ascertainment of functional differences in subpopulations activated by specific stimuli, (c) evaluation of neuronal activity in complex areas containing multiple neuroendocrine systems, (d) identification of other brain areas activated in conjunction with neuroendocrine systems following specific stimuli, (e) analysis of connectivity of activated neuroendocrine systems with other parts of the brain, and (f) identification of stimuli that decrease neuronal activity. The neuroendocrine systems studied include those that secrete arginine vasopressin (AVP), oxytocin (OT), corticotropin-releasing hormone (CRH), luteinizing hormone-releasing hormone (LHRH), and dopamine (DA). The use of c-Fos expression has permitted functional neuroanatomical mapping of these systems in response to specific stimuli such as cholecystokinin (CCK), hyperosmolality, and volume depletion, or during various physiological states such as the proestrous ovulatory luteinizing hormone (LH) surge and lactation. Although the use of c-Fos as a marker of neuronal activation will continue to be an extremely powerful technique, future studies will also be directed at relating immediate early gene expression to changes in neuroendocrine gene expression. To this end, we have shown that both c-Fos and c-Jun are expressed in neuroendocrine neurons in response to a number of stimuli, setting the stage for potential regulatory drive to genes containing AP-1 binding sites.