Inhibition of ion transport by exogenous prostaglandins (PGs) has previously been accomplished only by using “pharmacologic” doses (10−7–10−4 M). Therefore, experiments were performed in which the in vitro effects of low (physiologic) doses (10−11–10−7 M) were assessed in the Ussing chamber preparation of human jejunal mucosa, both with and without blockade of endogenous PG-production by indomethacin (2.9 × 10−5 M). Serosal application of PGE2 caused a prompt, dose-dependent rise in short-circuit current (Isc), decreased net Na flux (JnetNa), elicited Cl secretion, and slightly depressed conductance (G). Dose-response curves for the early effects on Isc and JnetNa were performed. The percentage maximal response (Rmax) and the concentration for half-maximal response (km) were determined by nonlinear regression analysis using the normal Michaelis-Menten function (iterative evaluation by “least squares”). In the presence of indomethacin, the threshold concentration for effect of PGE2 was 10−10 M; Rmax: stimulation by 26% and inhibition by 58% of control values; and km: 10−9 M and 0.7 × 10−9 M for Isc and JnetNa, respectively. In the absence of indomethacin, the threshold concentration for exogenous PGE2 was 10−9 M; Rmax: stimulation by 15% and inhibition by 46% of control values; and km 4.7 × 10−8 M and 1.2 × 10−8 M for Isc and JnetNa, respectively. The mean concentration of endogenous PGE2 released into the serosal bath was approximately 2 × 10−11 M for indo-methacin-blocked tissues, and 1.8 × 10−9 M for unblocked tissues. The clearance of exogenous PGE2 from the serosal bath was about 20% during the experimental period, while less than 1% diffused across the membrane into the mucosal bath. The initial rise in Isc rapidly declined so that PGE2 had a transient effect that cannot be explained by clearance of PGE2, but is rather probably due to the activation of self-controlling feedback mechanisms in the tissue. Since Jm−sNa and G during control periods were higher with indomethacin blockade than without, and since Isc during experimental periods with high PGE2 concentrations usually reached a stable level above baseline before the effect dissipated, PGE2 appeared to have a “steady-state” effect as well, although less sensitive. Similar responses in Isc and JnetNa, were observed following mucosal PGE2 application, but required concentrations larger than 10−8 M. PGF2α had no effect on ion transport up to a concentration of 10−7 M. In conclusion, suppression of PG-biosynthesis by indomethacin increased the initial sensitivity of the tissue to exogenous PGE2. The extremely low dose-response suggests a “true” physiologic role for PGs in the modulation of intestinal ion transport. Since km of the response to PGE2 appeared to be below the threshold for activation of the intestinal adenylatecyclase-cAMP system, the role of cAMP in physiologic responses to PGs remains to be established.