We recently showed that acute muscle pain, induced by bolus intramuscular injection of hypertonic saline, causes a sustained increase in muscle sympathetic nerve activity (MSNA) and a modest increase in blood pressure and heart rate1 . However, it is not known whether long-lasting (tonic) pain - which more closely resembles chronic pain - causes a sustained increase in MSNA and blood pressure. We tested this hypothesis by recording MSNA in 12 healthy subjects. Tonic pain was induced for ~60 minutes by slow intramuscular infusion of hypertonic saline (7%) into the ipsilateral tibialis anterior muscle. Pain was sustained at a tolerable level (5-6/10 on a visual analogue scale). Seven subjects showed progressive increases in mean MSNA amplitude during tonic pain, increasing to 154 ± 17% (SEM) at 45 mins and remaining essentially constant for the duration of the infusion. In these subjects blood pressure and heart rate also increased. Conversely, for the other 5 subjects MSNA showed a progressive decline, with a peak fall of 67 ± 11% at 40 mins; blood pressure and heart rate also fell in these subjects. We conclude that tonic muscle pain has long-lasting effects on the sympathetic control of blood pressure, causing a sustained increase in some subjects yet a sustained decrease in others. This may have implications for individual differences in the cardiovascular consequences of chronic pain. Given that some patients with post-surgical chronic pain go on to develop chronic pain whereas others do not2, we believe that our results may go towards explaining why this occurs.