In acute pantothenic acid deficiency in young rats, decreased growth of the tibia occurred, with marked impairment in chondrogenesis, osteogenesis and hematopoiesis. Four stages of morphologic changes were observed (1) at ages of 21-33 days, unusual trabecular resorption, decreased osteoblast proliferation and edema of the bone marrow were noted; (2) at 33-63 days of age marked retardation of osteogenesis, with some calcification in the epiphysis and blunt diaphyseal trabeculae was observed; (3) at 45-72 days, epiphyseal cartilage calcification began and osteogenesis ceased; (4) at 76-109 days, trabeculae were completely absent and a heavy layer of sealing-off bone formed below the epiphyseal cartilage. The role of pantothenic acid and of additional factors such as pteroylglutamic acid, unknown vitamins and calorie restriction in these histologic changes is discussed.