PONV after intrathecal morphine, occurs up to 30-40 percent. The patients having TKR normally are female, obese and non-smoker which are risk factors for PONV. Recently, a multimodal approach, combining several means to minimize PONV, has found wide acceptance as a standard of care. In our hospital, most of the patients are supposed to fast after midnight The oral rehydration therapy reduce patients thirst and increase his satisfaction, but whether or not this method can reduce the incidence of PONV after low dose (0.2 mg) intrathecal morphine is not investigated yet. The aim of this study is to investigate the effect of preoperative oral rehydration therapy on the incidence of PONV.