SUMMARY: Pregnant goats hypophysectomized between the 38th and 120th days of the gestation period aborted 3–9 days after operation. Pregnant goats subjected to section of the pituitary stalk between the 44th and 129th days of gestation aborted between the 6th and 17th days after operation. On the other hand, section of the pituitary stalk in pregnant sheep between the 42nd and the 90th days of gestation did not interfere with the pregnancy. In both goats and sheep the operation of stalk section included the insertion of a plate to form a barrier between the cut ends of the stalk. The only goats which did not abort were two of the hypophysectomized group in which some anterior lobe tissue, adjacent to the stalk, had not been removed, and one stalk-sectioned goat in which the plate had moved after insertion, allowing the cut ends of the stalk to join up.