Net ion and H2O transport by jejunum adjacent to the ligament of Treitz (proximal jejunum) and midjejunum were measured in vivo by continuous perfusion with HCO3-Ringer solution containing a volume marker. Proximal jejunum secreted Na and H2O, whereas midjejunum absorbed Na and H2O. Both segments secreted CO2 and absorbed K and Cl. D-glucose stimulated absorption of Na and H2O and the transmural electrical potential difference (PD) in both segments, but these changes were not accompanied by alterations in Cl, CO2, or K fluxes. However, the increse in Na absorption caused by 3-O-methylglucose was matched by an increase in Cl absorption. This, in addition to increased tissue lactate concentration after addition of D-glucose, suggests that organic anion maintains electroneutrality for Na transport enhanced by D-glucose. Cholera toxin had no effect on ion transport or PD in proximal jejunum, but cholera toxin stimulated secretion and increased the PD in more distal jejunum. Although proximal jejunum shows spontaneous secretory activity, its capacity for secretion is not as great as more distal small intestine.