While it has never been shown that warming fluid to body temperature prior to using it for amnioinfusion in labor is necessary, the practice is generally accepted. Ideally it is done with a blood warmer. Since blood warmers are expensive and not always available, fluid bags are often warmed in "constant temperature" devices used to heat blankets and fluid used in surgery. These units are ubiquitous, create no extra expense with their use and are a reasonable alternative to blood warmers. A study was designed to determine whether warming ovens actually did heat the fluids used for amnioinfusion to around 37 degrees C. Fluid bags were placed in the warming oven for 48 hours or more, and opening temperatures of the contained fluids were recorded. The temperatures were extremely variable, ranging from 21 degrees C to > 50 degrees C. The variability in opening temperatures was a result of wide temperature fluctuations in the warming oven itself and the condition of the fluid bags on removal. Blanket and surgical fluid warming ovens are not appropriate for heating fluids used in amnioinfusion during labor.