This study examined the influence of aerobic fitness on the responses of selected hormones to the combined stressors of sleep deprivation (SD) and sustained mental work. Six aerobically high fit (HF) (VO2max greater than 50 ml.kg-1.min-1) and six average fit (AF) (VO2max less than 40 ml.kg-1.min-1) female subjects were subjected to a period of sleep loss of 60 h during which time they performed sustained mental tasks with no physical activity component. Venous blood samples were drawn every 12 h at 1330 hours and 0130 hours and plasmas analyzed for cortisol, growth hormone (hGH), prolactin, thyroxine (T4), triiodothyronine (T3), and reverse-triiodothyronine (rT3). For cortisol, both the HF and AF groups exhibited the normal high-daytime and low-nighttime pattern of secretion, with levels increasing significantly as the duration of SD increased. The normal elevations of hGH and prolactin levels during normal sleep were suppressed during SD. No significant fitness effects were found for cortisol, hGH, and prolactin responses. Plasma levels of T4, T3, and rT3 increased significantly during SD, with highly fit subjects exhibiting higher levels of these hormones than those of average fitness. We suggest that aerobic fitness may influence the peripheral metabolism of T4 during SD, but that aerobic fitness does not influence the regulation of the classical stress hormones during SD.