Crystalloid solutions for intravenous use have complex physiologic effects. The acid-base approach of the late Peter Stewart is useful in analyzing some of these effects. The difference between the sum of the strong cations and the sum of the strong anions (strong ion difference) is an independent controlling factor of acid-base. Water acts as an acid, dissociating to supply hydrogen ions. Infusion of water induces a metabolic acidosis through a reduction in the strong ion difference. Dextrose infusion will also produce acidosis, which may be modified by the biochemical effects of glucose metabolism. Normal saline solutions produce perioperative acidosis through a reduction in strong ion difference. The acidosis is greater than the acidosis produced by solutions such as Ringer's Lactate, which contain multicarbon anions. Intracellular uptake and metabolism of the multicarbon anions lead to alkalosis. All crystalloid infusions induce alkalosis by dilution of plasma weak acids, particularly albumin. The total concentration of weak acids is another independent controlling factor in the Stewart approach. Ringers's Lactate also reduces plasma osmolality. Polyvinylchloride containers leach contaminants, such as phthalate derivatives, into the solutions. The physiologic importance of these effects is unclear.