Acute hypoxia inhibits, rather than stimulates, fetal breathing movements (FBM), but there has been controversy as to the activity and role of the peripheral arterial chemoreceptors in the regulation of breathing movements in the unanesthetized fetus in utero. However, after midcollicular brain stem transection or lateral pontine lesion, hypoxia causes FBM to become continuous and stimulated in rate and depth. To determine whether this stimulatory response involves peripheral chemoreceptors, we used a two-stage approach to examine the response to hypoxia after peripheral chemodenervation in lateral pontine-lesioned fetal lambs. The lateral pons was lesioned at 119-121 days, and the response to hypoxia was tested in the unanesthetized fetus 4 days afterward. Fourteen fetuses in which hypoxia stimulated FBM underwent either peripheral chemodenervation or sham denervation in a second operation. Hypoxia had no effect when the fetus was tested 4-5 days after peripheral chemodenervation, and the basal incidence of FBM was significantly lower. The stimulatory response was unchanged by sham denervation. We conclude that the peripheral chemoreceptors are active in fetal life and that they mediate the stimulation of FBM seen in response to hypoxia after removal of the lateral pontine inhibition. In addition, after pontine lesion there is evidence of tonic chemoreceptor-mediated influences on FBM, which are normally overriden in the intact fetus.