Frontal hypothalamic deafferentation (FHD), which disconnects the anterior hypothalamus from the preoptic area, stops the twice daily surges of prolactin secretion of pregnancy or pseudopregnancy in the rat, and causes rapid luteolysis. Medial hypothalamic deafferentation (MHD), which separates the anterior from the posterior half of the hypothalamus, does not interrupt pregnancy and causes a significant increase in the size of the corpora lutea. To see whether MHD induces an increase in the basal level of prolactin secretion and/or a change in the pattern of prolactin surges, pseudopregnant rats were subjected to MHD or a sham operation on day 3 (day 1 = day of oestrus) and their bloods assayed for prolactin on either day 5 or days 7--8. MHD caused a specific disappearance of the day-time prolactin surge and a diminution in the height of the night-time surge, but no change from the controls in the basal prolactin level. In spite of what thus appears to be a lesser secretion of prolactin than in the controls, the corpora lutea of the MHD rats were larger, and progesterone was secreted at a higher rate and for a longer time, than in the controls. The relation of these findings to the existence of a "surge centre" in the hypothalamus was discussed.