Various chronic diseases are characterized by persistently elevated sympathetic activity. Baroreceptors, chemoreceptors, and osmoreceptors are three of the many mechanisms participating in the control of sympathetic outflow. The purpose of this study was to investigate the relation between peripheral chemoreceptors and osmoreceptors in the control of muscle sympathetic nerve activity (MSNA) in humans. We hypothesized that increased plasma osmolality would augment the MSNA response to peripheral chemoreceptor stimulation. Thirteen healthy subjects (28 ± 2 yr) completed the protocol. Increased plasma osmolality was induced experimentally via an acute infusion of hypertonic saline, and peripheral chemoreceptor activation was elicited by end-expiratory apnea. MSNA was directly measured using the technique of peroneal microneurography. Heart rate (ECG) and blood pressure (Finometer) were also assessed. Plasma osmolality significantly increased from pre-infusion to post-infusion (284 ± 1 to 290 ± 1 mOsm/kg H₂O, P<0.001). Burst frequency (17 ± 2 to 22 ± 2 bursts, P=0.001) and total sympathetic activity (54.8 ± 7.2 to 75.0 ± 13.9 AU, P=0.039) during apnea both significantly increased from pre-infusion to post-infusion. There was no difference in the change in mean arterial pressure following the infusion (24 ± 3 to 23 ± 3 mmHg, P=0.563). In conclusion, elevated plasma osmolality enhanced the MSNA response to endexpiratory apnea in humans.