Plasma concentrations of arginine vasopressin were determined by radioimmunoassay in 13 normal children, 21 patients with febrile illnesses (not bacterial meningitis), and 17 patients with bacterial meningitis. The mean +/- 1 SD concentrations of AVP in the normal children and patients with various febrile illnesses were 0.7 +/- 0.6 and 1.0 +/- 1.2 muU/ml, respectively. The mean +/- 1 SD concentration of AVP in patients with bacterial meningitis was 3.3 +/- 2.3 muU/ml. The concentrations of AVP in the patients with bacterial meningitis were significantly greater (P less than 0.001) than those noted in the normal children or children with other febrile diseases.