SUMMARY: Potentials were recorded in the pituitary stalk of ewes after electrical stimulation of the inguinal nerves or dilatation of the vagina. The modifications provoked by destruction of sensory pathways at different levels in the central nervous system were studied with a view to defining the tracts which carry the impulses to the pituitary stalk. The potentials obtained with bipolar concentric electrodes were characterized by long latency and a negative wave of low amplitude and long duration. The apparent negativity of the response was due to the recording conditions. At the spinal level the tracts are bilateral but primarily contralateral. The dorsal tracts do not seem to be necessary for the transmission of impulses to the pituitary stalk and the nerve fibres are distributed diffusely in the lateral and ventral parts of the spinal cord. At the supraspinal level, the lemniscal system does not seem to be involved in this transmission. On the other hand, the passage of impulses needs the integrity of the mesencephalic reticular formation. The long latency of the responses of the pituitary stalk to electrical stimulation of the inguinal nerves was due to the transmission time of the impulses between the mesencephalic level and the pituitary stalk. This long latency does not originate in the periphery. The cause has not been determined but relay in the amygdala nuclei or in the cortex has been eliminated. The relationship between the potentials evoked in the pituitary stalk by the stimulation of the inguinal nerves or by vaginal dilatation and the release of oxytocin is discussed.