It has been suggested that ageing, or at least some of its symptoms, are related to a physiological decline in growth hormone (GH) levels with age. To elucidate the molecular mechanisms behind GH-dependent changes in aged animals, we have analysed GH-mediated effects on gene expression in old male rats through the application of cDNA representational difference analysis in combination with RNase protection assays and DNA macroarrays on nylon filters. In addition to well-characterized GH-regulated genes, we demonstrate the differential expression of at least 11 genes not previously known to be under GH control. Several transcripts encoding enzymes and receptors involved in the metabolism of protein, carbohydrates and lipids were identified. Another group of transcripts consisted of gene products encoding proteins involved in the immune system. This list of expressed genes in GH-treated old rats may shed further insight on the action and mechanism behind the positive effects of GH on, for example, body composition and immune responses observed in different animal and human studies.