Nitrous oxide (N2O) has analgesic and sedative properties but may potentially increase the incidence of PONV. N2O might increase the incidence of PONV by several potential mechanisms: (1) increase in middle ear pressure (2) bowel distension, (3) activation of the dopaminergic system in the chemoreceptor trigger zone and (4) interaction with opioid receptors. N2O has been demonstrated to increase the incidence of PONV in some studies but not in others. The present study is undertaken to evaluate the effect of nitrous oxide- free general anesthesia on the undesirable clinical outcome of PONV.