The isolated hindgut of larval Sarcophaga bullata is capable of secreting K ion, Na ion, NH4 ion, and Cl ion into the gut lumen under the appropriate conditions. The hindgut of Sarcophaga also maintains a transepithelial potential (TEP) difference such that the lumen of the gut is normally negative in respect to the hemolymph. On the basis of net ion fluxes and TEP measurements in the isolated hindgut of S. bullata, it was suggested that K ion, NH4 ion and Cl ion were actively secreted. Addition of small amounts of NH4 ion to the external medium resulted in a large decrease in K ion secretion and a concomitant large increase in NH4 ion secretion. In both cases, i.e., in the presence and absence of external NH4 ion, Cl ion secretion was maintained. These results were used to develop a model of the hindgut system in which Cl ion was constantly, and independently of cation movements, being secreted into the hindgut lumen. In addition, a cation pump was thought to be present in the hindgut epithelium in which K ion and NH4 ion competed for the same transport site. In order to determine if this is the case, unidirectional ion fluxes across the larval, isolated hindgut were measured with the appropriate radioisotopes. The results of this study have shown that Na ion is distributed passively in this system while K ion and Cl ion are actively secreted into the hindgut lumen. In addition, NH4 ion is secreted independently of K ion into the gut lumen. Because NH4 ion is transported independently of other cations and because of the relatively simple structure of the gut, the isolated hindgut of S. bullata represents an ideal model system in which to investigate the active transpot of the NH4 ion.