Previous studies show an inability of the skeletal muscle cell and red blood cell to maintain sodium, potassium, and calcium homeostasis during hemorrhagic shock in adults. However, there is no information on the cellular effects of shock in the neonate. This study examined the effects of hemorrhagic shock on red cell membrane function in a newborn canine model. Changes in sodium, potassium, calcium, and magnesium concentrations in red blood cells and plasma were correlated with changes in intracellular and plasma ATP levels. Newborn dogs (n= 36), 10 to 14 days of age and weighing 501 to 707 g, were studied. After baseline studies (blood pressure, heart rate, temperature), electrolyte and ATP concentrations in red blood cells and plasma were measured. The dogs were then bled 40% of their estimated blood volumes. All parameters were measured after 1 h of shock. This shock model produced hypotension, bradycardia, and acidosis. The red blood cell sodium, calcium, water, and ATP content increased in shock, whereas intracellular magnesium fell. Red blood cell potassium levels, plasma sodium, and calcium concentrations were not significantly altered in shock, although plasma potassium and magnesium levels rose. Our data show that shock in the newborn disrupts cell membrane integrity. Intracellular accumulation of sodium and calcium occurred despite cellular ATP uptake, suggesting that high energy deficits are not the primary mechanism contributing to electrolyte imbalance in newborn shock. (Pediatr Res24: 438–441, 1988)