BACKGROUND: Under particular circumstances older persons may be vulnerable to developing mild, chronic hypohydration. In young adults, hypohydration has been shown to impair muscle endurance, power and strength. Muscle performance declines with aging, a condition known as dynapenia. How hypohydration impacts muscle performance in older persons remains unclear. In this study we examined this question, believing it may identify a factor exacerbating dynapenia. METHODS: One-hour after having been passively exposed to heat where either a body mass loss of 1% was induced or euhydration maintained with fluid ingestion, eight healthy, active older men (age: 68 +/- 5 yrs) completed an exercise testing session where indices of muscle strength (30-sec chair stand, grip strength, maximal isometric seated leg-press extension), endurance (seated leg-curl flexion + seated leg-press extension to exhaustion at 60% of 1-RM) and power (30-sec Wingate test) were assessed. RESULTS: Gastrointestinal temperature before exercise testing was not significantly different from that measured before heat exposure with neither hydration regimen. Magnitude-based inferential statistics indicate that, from a clinical perspective, the effect of hypohydration on 30-sec chair stand (-1.0 +/- 4.4%) is possibly harmful, for grip strength (-2.4 +/- 4.1%), lower limbs endurance (-7.5 +/- 11.2%) and anaerobic power (-3.9 +/- 4.3%) likely detrimental, and unclear with respect to maximal isometric lower limbs strength. CONCLUSION: Maintaining adequate hydration in older men is important, since hypohydration of only 1% body mass could impede muscle endurance, power and strength and, consequently, worsen the impact of dynapenia.