We have used recombinant type 2 adeno-associated virus to deliver the gene encoding green fluorescent protein into the central nervous system of adult rats. Gene expression, determined by fluorescent microscopy, was observed not only at the site of injection but also in axons following known neuroanatomical pathways. We have demonstrated a spread of enhanced green fluorescent protein from the paraventricular nucleus of the hypothalamus into the median eminence and neurohypophysis. Cell bodies containing enhanced green fluorescent protein were also visualized in the paraventricular nucleus contralateral to the side of injection. These findings suggest that gene transfer by recombinant adeno-associated virus could be used as a tool to investigate hypothalamic-pituitary interactions and, elsewhere in the central nervous system, to trace axonal pathways.