1. Rumen and caecal wall tissues were taken at slaughter from lambs varying in age from newborn to 11 weeks. The ability of these tissues to metabolize the short-chain fatty acids, acetic, propionic and butyric acid was compared with tissues from adult sheep. Ketone body production was measured.
2. The utilization of butyrate by the rumen wall in the newborn lamb was lower than in the adult, but exceeded the adult levels at 3 weeks of age and maintained this higher utilization to 11 weeks and probably longer. Ketone body production was negligible at birth but followed butyrate utilization closely thereafter.
3. The caecal wall in the newborn lamb utilized butyrate at a much higher rate than the adult sheep tissue. Foetal lamb caecal tissue utilized butyrate to the same extent as in the newborn lamb. Levels were, however, typical of the adult within a day or two of birth and showed no subsequent effect of age. Ketone body production was negligible at all ages.
4. Rumen development in milk-fed lambs slaughtered at 7 and 9 weeks of age was retarded anatomically and showed decreased capacity in the utilization of butyrate.
5. The utilization of acetate and propionate by rumen and caecal tissues showed no marked change due to age. Ketone body production from these acids was low.