Quantitative autoradiographic techniques were used to localize adenosine A1 receptors at the light microscopic level with the antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine [( 3H]DPCPX) in the brainstem of fetal sheep. Since adenosine has been proposed as a neuromodulator, which effects the depression of fetal breathing movements during hypoxia, attention was directed to respiratory neuronal areas. The highest density of A1 receptors in respiratory related groups was found in an area of the rostral ventrolateral medulla, which is ventral to the facial nucleus, caudal to the superior olive and lateral to the rostral inferior olive. Intermediate densities were seen in the medial and lateral parabrachial nuclei. Adenosine A1 receptor density was low in the areas of the nucleus of the solitary tract and the nucleus ambiguous. These data suggest that moderate hypoxia in the fetus may depress respiration by withdrawing a tonic stimulus at the level of the ventral lateral medullary chemoreceptors.