1. The Na, K, Cl and water content of faeces removed from different parts of the pig helicoidal colon were determined for 1-day-old and adult animals. Faecal Na, Cl and water content fell in both cases during passage of contents through the colon. K content increased in the distal colon of the adult pig. This did not occur in the 1-day-old animal. 2. The colon of the 1-day-old pig removed a larger proportion of water from its contents than did that of the adult. The absorption of both water and Cl was found to extend into the distal colon of the 1-day-old animal; little or no net absorption took place in this region in the adult. 3. Colons taken from new-born pigs maintained stable short-circuit currents of about 60-80 muA cm-2 with open-circuit voltages of about 10 mV. Similar values were found for proximal and mid regions of colons taken from 1-day-old, suckled animals. In the distal colon, however, both short-circuit current and open-circuit potential doubled after suckling. 4. Measurements of Na flux in vitro showed no regional difference at birth. The amount of Na absorbed, about 4.5 muequiv cm-2 h-1, was twice that predicted from the short-circuit current, supposing that to be due solely to the electrogenic transport of Na. 5. Colons taken from suckled pigs transported Na at double the rate found in the new-born animal. This applied to both the proximal region, where the short-circuit current remained constant, and the distal region, where the short-circuit was double that of the new-born. 6. Fluxes of Cl and K were also measured across the proximal colon of the one-day-old, suckled pig. There was a net absorption of Cl and secretion of K (1.3 and -0.05 muequiv cm-2 h-1 respectively). These fluxes, taken together with that for Na, could not wholly account for the short-circuit current measured across these preparations. 7. The pig colon seems well able to cope with both electrolyte and water absorption during the first 24 hr of post-natal life. Part of the absorbed sodium appears to follow a non-electrogenic, possibly pinocytotic, route, but the full ionic contribution to the measured short circuit current has still to be determined.