Fluctuations in fetal hemodynamic parameters are commonly observed, and we can use the variation in the fetal heart rate as an index in fetal assessment. The purpose of this study was to clarify the spontaneous rhythm in systemic circulation in fetal lambs by means of power spectral analysis. Three pregnant sheep and their fetuses at 125 and 135 days of gestation were surgically instrumented and studied. After a minimum recovery period of four days, a fetal electrocardiogram was taken and arterial blood pressure was recorded. They were also examined in the absence of heart rate variation by means of fetal cardiac pacing. We used an autoregressive model to estimate their power spectral densities. In normal fetal lambs, two spontaneous rhythms were detected in the spectra for the fetal heart rate and arterial blood pressure. Spectral analysis of arterial blood pressure variation in the absence of heart rate variation revealed that there was only one consistent major spectral component around 0.15 Hz. This suggests that fluctuations are caused not by a variation in the heart rate but by a variation in peripheral motor activity. This spontaneous rhythm in peripheral resistance may be associated with the sinusoidal heart rate pattern in the fetal cardiotochogram.