Although the heart rate variability (HRV) response to hypoxia has been studied, little is known about the dynamics of HRV after hypoxia exposure. The purpose of this study was to assess the HRV and oxygen saturation (SpO(2) ) responses to normobaric hypoxia (FiO(2 ) = 9.6%) comparing 1 min segments to baseline (normoxia). Electrocardiogram and SpO(2) were recorded during a 10-min hypoxia exposure in 29 healthy male subjects aged 26.0 +/- 4.9 years. Baseline HRV values were obtained from a 5-min recording period prior to hypoxia. The hypoxia period was split into 10 non-overlapping 1-min segments and time domain HRV indexes (RMSSD and SDNN) were calculated for each segment. Differences (Delta) from baseline values were calculated and transformed using natural logarithm (Ln). This study revealed that the decrease in DeltaSpO(2) became significant (P<0.001) in the first minute of hypoxia, the decrease in DeltaLn RMSSD became significant (P = 0.002) in the second minute, and the decrease in DeltaLn SDNN became significant (P = 0.001) in the third minute. Between the second and fifth minute of hypoxia, DeltaSpO(2) correlated with DeltaLn RMSSD (r = 0.57, P<0.001) and DeltaLn SDNN (r = 0.44, P<0.001). Five min after the onset of hypoxia, DeltaSpO(2) was significantly (P = 0.002) decreased but changes in DeltaLn RMSSD (P = 0.344) and DeltaLn SDNN (P = 0.558) were not significant. In conclusion, the decrease in HRV was proportional to desaturation but only during the first 5 min of hypoxia.