Hen coprodeum absorbs sodium electrogenically and, when stimulated by theophylline, secretes chloride. In this study the vibrating microprobe technique was used to localize the transport of these ions to intestinal villi/folds and crypts. With the isolated, stretched epithelium, controlled by light microscopy and scanning electron microscopy, in open circuit, currents were inward, 40 +/- 7 microA/cm2, 50 microns vertically above villi, and outward, 36 +/- 7 microA/cm2 above crypts. The currents decayed exponentially to near zero at 300 microns with the same length constant. A physical model simulating the observed loci of current sources and sinks predicts potential profiles consistent with our data. Extrapolation of the currents gives a surface potential of 45 microV, negative on villi and positive above crypts. Short circuiting increased villus current to 86 +/- 27 microA/cm2 at 50 microns, and amiloride treatment reduced it to -8 microA/cm2; in both cases crypt currents were abolished. The inward currents are compatible with sodium absorption. Induction of chloride secretion after amiloride treatment, resulted in current circuits similar to those induced by sodium absorption, with villus currents of 23 +/- 7 microA/cm2. This is in accord with chloride secretion at the villi. Quantitative estimates of crypt number (860/cm2) and opening diameter (15 microns), in conjunction with isotopic measurements of active and electrical potential-driven ion fluxes demonstrate, however, that only 4% of the potential-driven co-ion transport occurs through the crypts. This indicates that nearly all chloride secretion comes from the sodium-absorbing villar area. Were the chloride secretion to occur solely from the crypts, the current should have been in the opposite direction and 10,000-fold larger.