Previous research has observed a decrease in reflex excitability as subjects move from lying or sitting to a standing position. This decrease is thought to be due to increased presynaptic inhibition at the Ia synapse (Katz et al. 1988; Koceja et al. 1993). It is not known whether these same reflexes are modulated in a similar fashion within naturally occurring postural sway during quiet standing. H-reflex and M-wave recruitment curves were obtained in the right soleus (SOL) and medial gastrocnemius (MG) muscles while human subjects (n=7) were in a standing position. Electrical stimuli (1 ms duration pulse between 0.5 to 14 mA) were delivered to the tibial nerve when the body was naturally swaying forward (centre of pressure (COP) at 1.6 standard deviations (SD) anterior to the mean baseline COP) or backward (COP at 1.6 SD posterior to the mean baseline COP). The two sway conditions were characterized by a difference of 0.5 deg, 1.1 cm, and 0.8 cm in the mean ankle angle, position of the centre of pressure, and position of the centre of mass, respectively. Compared to when swaying backward, forward sway resulted in a significant increase in Hmax for the SOL and MG muscles by 11.2 ± 8.6% and 19.7 ± 13.1% (mean ± SD), respectively. No differences were found in the Mmax for SOL and MG between sway conditions. The greater Hmax for the two muscles during forward sway may have been due to the concomitant increase in motoneurone excitability, as reflected by the significant increase in background electromyographic (EMG) activity of the SOL and MG (19% and 121%, respectively). While the increase in the SOL Hmax during forward sway was comparable to the increase in background SOL EMG activity, the increases in MG Hmax and MG EMG activity were disproportional. This was verified by a significant reduction (42%) in the MG Hmax:EMG activity ratio during forward sway. An explanation for the less than expected growth in MG Hmax during forward sway may be an increase in presynaptic inhibition, thereby reducing the efficacy of the Ia synapse. Such an increase of inhibition may be a result of increased inhibitory activity from primary afferent depolarization interneurones, or lie within the synapse itself due to increased Ia-afferent input through muscle lengthening (Wood et al. 1996; Pinniger et al. 2001). We conclude that reflex excitability is differentially modulated for the SOL and MG muscles within the natural sway parameters of upright stance and that the observed differences between these muscles imply a functional divergence during quiet standing.