Introduction: This study was conducted against a background of the following four points: a) increased drinking behavior in children with attention-deficit hyperactivity disorder (ADHD), b) the parallel between some behaviors associated with ADHD and with 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 the studies of hypertension and drinking, Methods: Measures of NPY, catecholamines and electrolyte balance were compared in the plasma and urine of healthy children and those with ADHD. Drinking was monitored during three hours of neuropsychological tests over two days in 14 ADHD (mean 9.8 years-of-age) and 9 healthy children (10.6 years-of-age). Results: Patients drank 4 times more water and showed twice the levels of NPY 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. NPY levels were inversely related to calcium excretion and drinking inversely to sodium circulating (but positively with potassium and phosphate excretion). Conclusions: Increases of drinking and increased levels of circulating NPY in ADHD children and decreased electrolyte excretion may reflect a common disturbance in the homeostatic control of metabolism.