To determine the relationship between abnormal fetal heart rate (FHR) patterns, fetal oxygen content by reflectance pulse oximetry, and the effects of maternal oxygen administration, 158 intrapartum women consisting of 120 women with normal FHR patterns, and 38 women with varying degrees of variable FHR decelerations were examined. A new reflectance pulse oximetry probe was attached to the fetal forehead to continuously monitor fetal oxygen saturation (SpO2) during labor. Oxygen was administered for 30 min at 5 liters/min by nasal canula in 32 women. No significant change in fetal SpO2 was seen in women with normal FHR patterns, but a significant decline in SpO2 was observed shortly before births. FHR decelerations less than 90 bpm occurred prior to the decline in fetal SpO2 Maternal oxygen administration was effective in increasing fetal SpO2 in fetuses with decreased SpO2 (SpO2 < 50%), but not in fetuses with high SpO2 (SpO2 > 60%). It is concluded that fetal SpO2 exhibits no change during normal labor but significantly declines shortly before birth, that relatively severe variable decelerations (< 90 bpm) can decrease fetal SpO2, and that maternal oxygen treatment at 5 liters/min is effective in augmenting fetal oxygen tension in fetuses with decreased oxygen tension.