Replication deficient, recombinant adenoviruses (Ads) have been used successfully to transfect several forebrain and brainstem nuclei, but have yet to be demonstrated as useful vectors for transgene delivery in the structurally diverse and highly vascularised nuclei of the hypothalamus. In the present study we have assessed the ability of an Ad expressing the lac-Z gene to transfect cells of the paraventricular nucleus (PVN) of the hypothalamus in vivo. We show that: (1) we can achieve stable expression of the lacZ gene in cells of the magnocellular PVN for at least 2 months; (2) there were no obvious differences in the level of AVP mRNA in the PVNs injected with Ad compared with those injected with vehicle suggesting that Ad treatment is not disrupting normal cellular function in the injection region; (3) the introduction of Ads results in a limited immune response; (4) systemic treatment with cyclosporin dramatically reduces its magnitude. We conclude that Ad vectors represent useful tools for neuroendocrinological and gene therapeutic studies of the hypothalamus.