The purpose of this study was to determine the effects of a lateral turn on mixed venous oxygen saturation (SvO2) and heart rate in 183 critically ill adults. Mean SvO2 decreased from a baseline of 67% to 61% saturation (p less than 0.0001) immediately after turning and gradually returned to 66% saturation (p less than 0.002) within 4 minutes. Mean heart rate increased slightly from a baseline of 99 beats/min to 102 beats/min (p less than 0.0001) immediately after turning and decreased slightly to 101 beats/min (p less than 0.0004) within 4 minutes. These statistically significant changes in SvO2 and heart rate were not clinically significant for most patients. However, physiologic responses to turning were highly variable. SvO2 reductions of 25% or more from baseline, heart rate increases and decreases of 10 beats/min or more, and signs of activity intolerance occurred in some patients. Nurses should expect critically ill patients to have a decrease in SvO2 of approximately 9% of baseline and small changes in heart rate after turning. These changes should be transient, with SvO2 and heart rate gradually returning toward baseline levels during the next 4 minutes. If turning triggers large or prolonged changes in SvO2 or heart rate, prompt repositioning and evaluation are needed to prevent adverse effects.