The physiological significance of the spectral and fractal components of spontaneous heart rate (HR) variability in the fetus remains unclear. In order to examine the relationship between circadian rhythms in different measures of HR variability, R-R interval time series obtained by fetal electrocardiograms (ECG) were recorded continuously over 24 hours in five pregnant sheep at 116 to 130 days gestation. Conventional measures of short-term variability (STV) and long-term variability (LTV), low- frequency (LF, 0.025~0.15 cycles/beat) and high-frequency (HF, 0.2~0.5 cycles/beat) spectral powers, LF/HF ratio and fractal dimension values were calculated from 24-hour ECG recordings and quantified every 60 minutes. STV, LTV, and LF and HF spectral powers were minimal during the day but increased significantly to their highest values at night. We found a significant positive correlation between these measures, while the cosinor method showed significant similarity between their circadian rhythm patterns. Fetal R-R intervals also exhibited fractal structures. Fetal heart rate variability had a fractal structure, which was similar between day and night. These results suggested that the circadian rhythms exhibited by STV and LTV during the day were mainly due to changes in frequency components rather than to fractal components of fetal HR fluctuation.