Against a background of (a) increased drinking behavior in children with attention-deficit hyperactivity disorder (ADHD); (b) the parallel between some behaviors associated with ADHD and hypertension; (c) the use of the spontaneously hypertensive rat as a model for ADHD; and (d) similarities in the changes of neuropeptide Y (NPY) and catecholamine in studies of hypertension and drinking, NPY, catecholamines and electrolyte balance were compared in the plasma and urine of healthy children and those with ADHD. Drinking was monitored during 3 h of neuropsychological tests over 2 days in 14 ADHD and nine healthy children. Patients drank four times as much water and showed twice the levels of NPY found in controls. In controls there were positive and in patients there were negative relationships for NPY with drinking and restless behavior. Patients' plasma levels of norepinephrine (NE) and epinephrine were slightly elevated, but urinary levels of NE and the serotonin metabolite were markedly increased. Urinary excretion rates for sodium (not potassium), phosphate and especially calcium were decreased in patients even after covarying for less urine production in the ADHD group. NPY levels were inversely related to calcium excretion and drinking was inversely related to circulating sodium. Increases of drinking and circulating NPY in ADHD children and decreased electrolyte excretion may reflect a common disturbance in metabolic homeostasis.