Age-associated atrophy of the thymus with coincident reduction in thymopoeisis, decline in thymic output, and subsequent immune dysfunction has been reversed by the use of interleukin-7 (IL-7). In the earlier studies and in clinical trials, delivery of IL-7 has been by multiple injections over several days to maintain effective activity levels in the tissues. This is unlikely to meet with high compliance rates in future clinical use, and so we tested alternate routes of delivery using a technique involving tagging IL-7 with fluorescent dye that emits in the near-infrared region and whose fluorescence can be visualized within the tissues of live animals. We have shown that intratracheal instillation, enabling transfer through the lungs, provides an effective route for delivering IL-7 into the bloodstream and from there into the tissues in older animals. Delivery is rapid and widespread tissue distribution is seen. Comparison of administration either subcutaneously or by instillation reveals that IL-7 delivery by the pulmonary route provides significantly greater transmission to lymphoid tissues when compared with injection. In functional assessment studies, pulmonary administration led to significantly improved intrathymic T cell development in older animals when compared with IL-7 delivered by injection. Furthermore, in these older animals, delivery of IL-7 by intratracheal instillation was not accompanied by any apparent adverse events when compared with controls receiving saline vehicle by instillation or animals receiving IL-7 by subcutaneous injection.