Clinically undetected underhydration lowers subcutaneous tissue oxygen, impairs healing and increases wound infection. This study examined the effect of supplemental oral hydration on noninvasive measures of skin temperature (T(s)), transcutaneous skin O(2) (PtcO(2)), and CO(2) (PtcCO(2)), and skin blood flow (laser Doppler fluxmetry [Flux]=concentration of moving blood cells [CMBC] x Velocity) in healthy adults. Nineteen Control and 18 Test (Hydrated) subjects participated. Ad libitum fluid intake was allowed before the experiment. Sensors were placed on the chest, lower legs, and feet. Time 1 resting supine T(s), PtcO(2)/PtcCO(2), and Flux/CMBC/Velocity were obtained. Then, the Test group ingested 500 mL of H(2)O. Time 2 measures were obtained 30 minutes later. Data were analyzed using ANOVA. Change in T(s), PtcO(2), PtcCO(2), and Velocity did not differ between groups. Flux increased more in the Test than in the Control group (p<0.01). The Flux change was explained by a CMBC increase (p<0.05). For midlife/older subjects only, the CMBC change was greater in the Test than in the Control group (p<0.01). Flux may be sufficiently sensitive to detect microvascular changes in response to supplemental oral hydration. These data also suggest that subtle levels of hypoperfusion due to underhydration may exist in community-dwelling healthy midlife/older adults in their natural state.