Wet weights, total bone alkaline phosphatase activity and specific alkaline phosphatase activity were determined on demarrowed femurs of normal, ‘hypophysectomized’ and pituitary-grafted chick embryos at selected intervals of incubation. In normal bones, all parameters noted above increased progressively through developmental time. ‘Hypophysectomy’ by means of surgical decapitation significantly retarded the normal increase of femur wet weight, total and specific alkaline phosphatase activity; in embryos bearing pituitary transplants, there was a return towards normal values. The possible role(s) of the pituitary in skeletal maturation and enzyme synthesis or activation is discussed.