Immature pregnant mice treated with glucocorticoid hormone (triamcinolone hexacetonide) were killed 1 week following delivery. The epiphyseal growth plates of the humerus and their cellular constituents were analyzed quantitatively. It became evident that pregnancy per se evokes a marked suppressive effect upon young cartilage cells, thus interfering with the normal process of endochondral bone formation. Pregnant mice treated with glucocorticoid hormone experienced an even higher degree of chondrocytic derangement, however, to a lesser extent than chondrocytes of nonpregnant mice treated with the hormone. It therefore appears that pregnancy-induced elevation of systemic estrogens and progesterone provides some kind of a protection against the antianabolic activity of glucocorticoids upon chondrocytes.