The possibility that 24-hour rhythms exist in uterine blood flow (UtBF) and umbilical blood (UmBF) was investigated in 5 days postoperative chronically instrumented near-term pregnant sheep acclimated to a controlled environment. UtBF, UmBF and pressure measurements were made at 15-min intervals over 24 h beginning at 0800 h. Each data series was examined for the presence of significant rhythms of a 24-hour period using a method of Fourier analysis. UtBF 24-hour rhythms were found in all ewes; UmBF 24-hour rhythms were found in 4 of 5 lambs. A consistent reciprocal phase relationship between UtBF and UmBF was evident within animals. There were no associated rhythms in maternal arterial, fetal arterial, or amniotic fluid pressures. These results indicate that the presence of circadian rhythms must be considered as a possible variable when long-term uteroplacental hemodynamic studies are planned.