The role of peripheral chemoreceptors in the circulatory adaptation to hypoxia and the effects of rapid-eye-movement (REM) and non-REM (NREM) sleep and breathing activity on organ blood flow were assessed in fetal sheep. Blood flow was measured with isotope-labeled microspheres on intact, vagotomized (VX), and sinoaortic-denervated (SAD) fetuses. Denervation did not change the biventricular cardiac output (Biv. CO) or organ blood flows during normoxia. In intact fetuses the blood flow was increased during hypoxemia in brain, adrenals, and heart but not in kidneys, skeletal muscles, or placenta. The increase in organ blood flow during hypoxemia was reduced in the VX group and even more in SAD fetuses, but in the latter group, blood flow was still increased in mid-brain, medulla, pons, skeletal muscles, and heart. Sleep states per se did not significantly affect the blood flow to any organs tested. However, the Biv. CO and blood flow to all organs except kidneys and adrenals was increased during fetal breathing in REM sleep. We conclude that 1) during moderate hypoxemia both aortic and carotid bodies plus an additional mechanism are involved in redistributing fetal blood flow, and 2) changes in organ perfusion during REM sleep are due to concomitant fetal breathing.