Objective: Hydration status is critical to physiological and cognitive health, yet it is unclear how populations living in hot-humid environments experiencing lifestyle transitions manage this underexplored facet of heat adaptation. This study assesses the predictors of repeated measures of hydration status for adults from two villages (close and distant from a market town) in the Bolivian Amazon. Materials and Methods: Interviews and focal follows were conducted with 36 Tsimane' (50% male). Urine samples, temperature, activity levels, and anthropometrics were measured pre-interview and post-follow and yielded a small panel (72 observations). Urine samples were analyzed for urine specific gravity (USG), a biomarker of hydration, with a refractometer. Results: The mean USG was 1.020 g/ml (SD +/- 0.008) with men (1.022 +/- 0.008) slightly more dehydrated than women (1.018 +/- 0.007). Using 1.020 as the criteria for clinical dehydration, 42% of the participants were dehydrated at both intervals and 21% were extremely dehydrated. Controlling for activity level, village membership, and covariates in random-effects linear regression models, each degree centigrade increase was associated with a USG increase of 0.0008 g/ml (P=0.000). Adults from the village closer to the market town had significantly higher USG (B=0.0041; P=0.04) than those in the distant village. Dehydration was predicted to occur at 29 degrees C, just above the thermoneutral range, and extreme dehydration at 37 degrees C. Discussion: These findings suggest that hotter temperatures coupled with lifestyle transitions may create conditions that increase vulnerability to dehydration among rural populations through landscape modifications and diet changes. (C) 2015 Wiley Periodicals, Inc.