Decreased secretion of growth hormone (GH) during aging may play a role in the impairment of muscle performance. To test this hypothesis we evaluated the effects of a chronic treatment with 150 micrograms/kg of GH on membrane electrical properties of extensor digitorum longus muscle of 23-month-old Wistar Kyoto female rats. We found that such treatment could partially but significantly reduce the age-related changes of these parameters. Indeed, with respect to untreated age-matched control rats we found a complete restoration of fiber diameter to the adult value, and a partial improvement of membrane resistance and capacitance values. Furthermore, the GH treatment significantly increased the low total membrane conductance characterizing aged muscle fibers. The increase of total membrane conductance was due to a significant increase toward the adult value of chloride conductance. In addition, the muscles from GH-treated aged rats had values of potassium conductance completely restored down to the adult ones. Accordingly, membrane excitability was also significantly improved by the treatment. The same treatment in adult female rats produced a slight but significant increase of both chloride conductance and potassium conductance, whereas in adult male rats we observed only a significant increase of fiber diameter. Our data show that impairment of GH secretion may contribute to the age-related changes of membrane electrical properties of skeletal muscle and support the potential benefit of GH administration to improve muscle performance in aged subjects.