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Insectivorous bats, which feed once daily, produce maximally concentrated urine only after feeding. The role of urea as an osmolyte in this process was investigated in pallid bats (Antrozous pallidus) in the laboratory. Following a 24-h fast, plasma and urine were sampled before and 2 h after feeding in postprandial (PP) animals and before and 2 h after similar treatment without feeding in nonfed (NF) animals. Food consumption by PP animals and handling of NF animals had no effect on blood water content as measured by hematocrit and plasma oncotic pressure. Food consumption increased both plasma osmolality (Posm) and plasma urea (Purea) by as much as 15%. Food consumption also increased urine osmolality (Uosm) and urine urea (Uurea) by 50–100%. Feeding increased Uosm regardless of changes in Posm, and elevation of Uosm resulted primarily from increased Uurea. In NF bats, Posm and Purea were unchanged, while Uosm and Uurea increased by as much as 25%. Again, increased Uosm resulted primarily from increased Uurea. The PP urine concentration cycle of pallid bats resulted from increased urea excretion in response to apparent rapid urea synthesis. Bats rapidly metabolized protein and excreted urea following feeding when body water was most plentiful. [Copyright &y& Elsevier]