OBJECTIVE: To determine fluctuation in mixed venous oxygen saturation in critically ill medical patients during a period of rest. DESIGN: Nonexperimental, descriptive. SETTING: The medical and coronary intensive care units in a large county hospital in south-central Texas. PATIENTS: Twenty critically ill patients, aged 19 to 85, who had placement of a pulmonary artery catheter capable of continuous monitoring of mixed venous oxygen saturation. The majority had a diagnosis of respiratory failure and required mechanical ventilation. METHODS: Mixed venous oxygen saturation was recorded each minute for a 2-hour period in either early morning or late afternoon hours. Minute-by-minute values obtained during a 30-minute period of rest were used to determine fluctuation. The lowest and highest mixed venous oxygen saturation values during the period were used to calculate percent changes from average values (fluctuation). RESULTS: The range of fluctuation was +/- 6% of the average mixed venous oxygen saturation value for 80% of the sample. Four patients had a greater range of fluctuation: however, their actual mixed venous oxygen saturation values were within a clinically acceptable range. No significant differences in percentage of low or high fluctuation were noted for the following variables: time of day, medication administration, oxygen delivery, oxygen consumption and average mixed venous oxygen saturation. The percentage of low fluctuation was significantly lower for four patients who were not mechanically ventilated. CONCLUSIONS: Knowledge of normal fluctuation enables the care giver to evaluate changes in mixed venous oxygen saturation in response to activities and/or treatments. Additional study of fluctuation in homogenous groups of critically ill patients is warranted.