Behaviour of the New-born Monkey. Observations on eleven new-born monkeys in the Carnegie colony of the Department oFEmbryology, Carnegie Institution of Washington, are recorded in the Year Book (No. 32, 1933). Additional data were derived from subsequent pregnancies in this colony and from one of another species of macaque born in the Yale colony. The full-term macaque baby opens and closes his eyes, cries, reaches out and grasps objects with his hands before he is completely delivered. At birth his flexor muscles are more precocious than his extensors, and their principal function appears to be the seeking of bodily support. By the second day there is a tendency to climb upward. By the end of the first week all the sensory mechanisms show evidence of functioning and the sensory-motor co-ordination develops much more rapidly than in the human infant. Play activities, such as romping, jumping, attempts to leap upon and seize objects, appeared during the second and third weeks but co-ordination of eye and hand and distance perception were quite imperfect. In the relationship of mother and baby much of the behaviour is subject to mechanistic explanation. Equipped with his grasping tendencies and the associated ventro-ventral position, the nosing and mouthing activities of the infant result in the discovery and seizure of the nipple, unaided by the mother. If the baby, through immaturity, lacks sufficient strength and co-ordination to accomplish these ends himself, the mother instinct is inadequate for the predicament, and he perishes.