Transient decrease of arteriel oxygen saturation (O2Sat) is reported in 5% tol5% of chronic renal failure (CRF) patients during hemodialysis (HD) procedure. Two main mechanisms have been proposed as the possible causes of this issue: on the one hand pulmonary leukostasis caused by the effecting of blood-dialysis membrane, and on the other hand, hypoventilation caused by the transfere of CO2 to acetate dialysate. Bronchoconstriction which occurs during HD has additional effect to hypoxemia. In the present study, we aimed to determine the changes of Peak Expiratory Flow (PEF) rates and the O2Sat that occur during HD and the relationship between these parameters. One hundred patients (mean age 40.38±13.12 years: range 10-67 years: 34 females, 66 males) who were in HD programme at Başkent University Hemodialysis Center between June 98 - September 98 were included in this study. The patients who develop CRF due to different etiologies were under HD programme for a period of 4 months-20 years (mean 5.04±4.2years). During dialysis, an average of 1-5 It (mean:2.87±0.8) of fluid was taken from patients. The dialysis machinary with the Cuprophan membrane and dialisate solution with bicarbonate were used during HD. The PEF rates and O2Sat were measured before dialysis and at the 30th, 60th, 180th minutes of the procedure. An increase of 1% (380-383) in the 30th and 60th minutes and 8% (383-409) in the 180th minute were observed, but there were no statistically significant differences in between them (p>0.05). Also, no significant changes were present in between the O2Sat values at the 0'", 30'", 60'", 180'" minutes of HD (p>0.05). These findings contradicts with the previous studies reported in the literature. Our results could be explained by the use of bicarbonate dialisate during HD and by taking fluids of >2lt which increase pulmonary permeability has additional positive effect in maintaining normal O2Sat levels.