Upper airway muscle dysfunction is implicated in obstructive sleep apnoea syndrome (OSAS), a common respiratory disorder associated with recurrent hypoxaemia. The prevalence of OSAS is higher in males than females. We tested the hypothesis that sex differences exist in the effects of intermittent hypoxia on upper airway muscle function. Adult Wistar rats were exposed to intermittent hypoxia (IH, 90 s air/90 s N(2); 5% O(2) at nadir) or sham treatment for 8 hours/day for 9 days. Following treatments, animals were killed humanely and isometric contractile properties of the sternohyoid (SH) muscle were examined at 35OC in vitro. Force-frequency relationship was determined at stimulus frequencies ranging 10-100 Hz. In male rats, SH peak force was decreased in IH-treated male rats [22.7 +/- 08. vs. 15.9 +/- 0.9 N/cm(2), sham (n = 8) vs. IH (n = 8), p < 0.001 ANOVA]. Conversely, in female rats, IH treatment had no effect on SH peak force [21.0 +/- 1.2 vs. 19.8 +/- 0.8 N/cm(2), sham (n = 8) vs. IH (n = 8), p > 0.05 ANOVA]. We conclude that IH-induced impairment of pharyngeal dilator muscle performance may contribute to OSAS.