Abstract 1. 1. When feeding on figs ( Ficus insipida), the bat Artibeus jamaicensis increases dietary sodium density while decreasing potassium density by primarily extracting and ingesting pulp juices rather than other parts of the fruit. 2. 2. Based on urine osmotic pressure, these bats are uniformly dehydrated when they leave day roosts and become rapidly rehydrated (0.5–1 hr) after initiation of feeding. 3. 3. After 2000 hr, and throughout the night there is no difference in urine concentration of free-flying bats compared with bats held in the laboratory without food or water for the same time interval. 4. 4. Mean maximum urine concentration in this species is 972 mOsm/kg.