• We measured urine vasopressin (VP) once daily on days 1 through 3 in 18 patients hospitalized with meningitis. Urine VP values were 215±100, 116±44, and 69±23 pg/mL on days 1 through 3, respectively, for children with bacterial meningitis and 34±14, 20±4, and 15±4 pg/mL for those with aseptic meningitis. Urinary VP levels of infants with bacterial meningitis were significantly greater than those of healthy ambulatory subjects (n =18) on all three study days; VP values of infants with bacterial meningitis were also greater than those of infants with aseptic meningitis on study days 2 and 3. The VP levels for the subjects with aseptic meningitis were significantly greater than those of the controls on day 1 only. None of the infants exhibited the clinical syndrome of inappropriate antidiuretic hormone secretion.