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We examined evaporative water loss of neonate (<1 yr old) tortoises in laboratory experiments designed to evaluate the dependence of evaporation oil humidity, and of juvenile (1-4 yr old) tortoises in field experiments designed to reveal the influence of burrow microclimate oil water gain and loss. in controlled laboratory conditions, rates of body mass loss which reflect net evaporative water losses, were independent of the difference in vapor density between the animal and its environment. Changes in skin permeability and respiratory parameters may, account for this. Sleeping tortoises lost body mass half as fast as did active tortoises and hibernating individuals lost body mass 1/20(th) as quickly as active animals. juveniles confined to short (20 cm) or long (70 cm) burrows in the field lost body mass faster in the drier and warmer short burrows. Doubly labeled water was used in tortoises residing in different burrow types to measure total (unidirectional) rates of evaporation, vapor influx and metabolic water production, to partition net water loss (as reflected in body mass loss) into its parts. Total evaporation rate was independent of burrow conditions, but tortoises in the longer, more humid burrows had higher rates of water vapor input and total water input than did those in shorter burrows. Thus, tortoises in long burrows lost body mass more slowly in response to a higher humidity, in contrast to neonates under laboratory conditions, Rates or body mass loss due to evaporation from neonates were relatively high in the laboratory (0.4 to 0.8 % of body mass per day) and the field (0.7 to 1.1 %/d compared to those, of adults in the laboratory (0.17 %/d) or the field (0.1 %/d). Thus, young tortoises apparently are obliged to rely oil behavioral means (drinking pooled rain, withdrawing into their shell, seeking long, deep burrows) to avoid lethal dehydration in relatively hot, dry seasons.