Background: The autonomic nervous system plays an important role in the control of fetal response to acute hypoxia and prolonged fetal distress. We wanted to find out if monitoring of the sympathetic tone by frequency-specific analysis of heart rate has clinical value in the prediction of fetal distress. The intrauterine fetal ECG was recorded in the acute and prolonged fetal distress and examined applying spectral analysis of heart rate. Subjects: 12 women at 35-40 weeks of gestation with no complications during the pregnancy were considered to have the acute fetal hypoxia for decelerations in CTG during the labour. The patients were classified into two groups according to the cord venous pH. Group 1 (n=5, pH<7.25) and Group 2 (n=7, pH>7.25). In addition, 10 women at 35-40 weeks of gestation were selected to Group 3 of prolonged fetal distress, because the pregnancies were complicated by hepatogestosis although there were no decelerations in CTG during the labour. Also 12 women at 35-40 weeks of gestation with no complications during the pregnancy formed Group 4. Some women in Group 4 had first degree decelerations in CTG during the labour. There were no differences between the groups in the sex of the fetus, birth weight. Apgar score or placental weight. Methods: The FECG was detected by an intrauterine scalp-electrode using a STANN® - fetal ECG monitor. The FECG was stored on a magnetic tape with an FM tape recorder and replayed using a voltage threshold trigger into a PC computer and the R-R intervals were measured by a Dasylab® - program. The frequency-specific heart rate variability (HRV) was quantified by Fast - Fourier- Transformed spectral analysis at three frequency bands:1) low-frequency (LF)0.03 - 0.07 Hz corresponding to the sympathetic control 2) mid-frequency band (MF) 0.07 - 0.13 Hz corresponding to the baroretlex mediated control and 3) high-frequency(HF) 0.13 - 1.0 Hz corresponding to the dominantly parasympathetic control of heart rate. The LF HF - ratio of spectral HRV densities was computed to estimate the symphatovagal balance. Results:1) In acute hypoxia(Group 1) the sympathetic tone was significantly increased in comparison with Group 2 (LF-band spectral density (p=0.04)). The LF HF -ratio of HRV was also increased in Group 1, but not significantly (p=0.08). 2) In prolonged fetal distress related to hepatogestosis (Group 3) the sympathetic tone of heart rate control was significantly increased when compared with Group 4 (LF-band spectral density (p=0.01). LF HF-ratio (p=0.03)).Conclusion: The activation of the sympathoadrenal system is of great importance in maintaining the compensatory mechanisms during fetal distress. LF components of HRV are an indicator of this activation. In our study the power spectral analysis of fetal HRV provided a method to assess quantitatively the sympathetic activation of the cardiac control of the fetus and to predict fetal distress.