Suckling rats were injected with thyroid and adrenal cortex hormones to explore their maturative effect on the activities of colon glycosidases. In experiment 1, rats 2, 5, and 8 days old were injected daily with thyroxine (2 micrograms/g body weight) or cortisone acetate (50 micrograms/g body weight) for 3 continuous days. Rats were sacrificed on the 4th day. In experiment 2, rats 4 and 11 days old were injected for 4 continuous days with T3 (0.1 or 0.5 micrograms/g body weight) or a synthetic analog, DIMIT (at the same dose), with sacrifice on the 5th day. Activities of beta-N-acetylglucosaminidase, beta-galactosidase, alpha-fucosidase, and beta-glucuronidase were assayed in homogenates of colon divided into two segments, proximal and distal. Administration of hormones did not influence the protein content of the colonic segments, but exhibited an effect on enzymatic activity, depending on age and on colonic segment. Specific activity of the first three enzymes was precociously decreased in the proximal segment in the younger rats (5 and 8 days old), but not in the older groups (11 and 15 days old). No effect of these hormones was noted on the activities of these enzymes in the distal segment.