The current trend in anaesthesia is to choose crystalloid over colloid fluids for volume replacement in the operating room. Outcome-oriented studies and kinetic analyses have recently provided more insight into how crystalloid infusions should be managed. These fluids have a much better short-term effect on the plasma volume than previously believed. Their efficiency (i.e. the plasma volume expansion divided by the infused volume) is 50-80% as long as an infusion continues, while this fraction increases to 100% when the arterial pressure has dropped. Elimination is very slow during surgery, and amounts to only 10% of that recorded in conscious volunteers. Capillary refill further reduces the need for crystalloid fluid when bleeding occurs. These four factors limit the need for large volumes of crystalloid fluid during surgery. Adverse effects associated with crystalloid fluids mainly include prolonged gastrointestinal recovery time, which occurs when > 3 L has been infused. Clinicians who do not want to prolong the length of the hospital stay by 1-2 days due to such problems may use colloid fluid selectively, but calculations show that the therapeutic window for colloids is quite narrow. Inflammation is likely to decrease the fluid efficiency of colloid fluids, while its effect on crystalloids is unclear. However, some recent evidence suggests that inflammation accelerates the turnover of crystalloid fluid as well.