The development of colonic water and electrolyte transport was studied in rats. The colon of 20-day-old (young) and 40-day-old (adult) rats was perfused with either a hypotonic (200 mosmol/L) or a hypertonic (450 mosmol/L) solution. The solutions had identical electrolyte contents. Polyethylene glycol was used as a marker of water absorption. Na,K-Adenosine triphosphatase (Na,K-ATPase) activity was determined in nonperfused colonic segments in 10- to 40-day-old rats. Water absorption was always higher in young rats than in adult rats. In the young rats but not in the adult rats increased osmotic pressure of the luminal fluid resulted in a significant decrease in water absorption. The secretion of K was observed only in adult rats. Na,K-ATPase activity increased significantly from 10 to 20 and from 20 to 40 days of age. The results imply that the immature colon has higher water conductivity and low active electrolyte transport.