Human muscle nerve sympathetic activity measured by microneurography during supine rest is known to vary considerably between healthy subjects, whereas in a given individual the level of muscle nerve sympathetic activity is stable over weeks and months. To further characterize long-term variability or reproducibility microneurographic recordings of muscle nerve sympathetic activity were performed in 15 healthy, normotensive subjects (mean age 51 years) who had undergone the same procedure between 10 and 14 years earlier (mean 12 years). The range of muscle nerve sympathetic activity was 9-59 in the first and 13-61 bursts/min in the second recording. Subjects maintained the level of muscle nerve sympathetic activity displayed previously, although with a slight but significant tendency to a higher outflow with increasing age. It is concluded that muscle nerve sympathetic activity is characterized by large inter-individual differences and strong intra-individual reproducibility over many years, with a tendency to increase with age. The age relationship is only in a minor part responsible for the variability, the cause of which remains unexplained. Because of the marked difference between individuals, strict normality criteria are difficult to define when comparing groups of subjects. There remains the risk of either obtaining spurious differences or obscuring a true abnormality. This is unlikely to apply when results in individual subjects are compared.