Studies were designed to examine the basis for the difference in molecular weights of the two proteins detected in membrane preparations of rat submaxillary glands after photolabeling with a radioactive analogue of substance P, 125I-p-benzoyl-L-phenylalanine8-substance P. When the two proteins were separated and individually digested with endoglycosidase F, the relative molecular weight of each protein was reduced by approximately 10,000, indicating that the extent of glycosylation of both proteins is the same. To test whether the difference in their molecular weights can be attributed to a difference in the lengths of the two proteins, photolabeled membranes were treated with carboxypeptidase Y before solubilization to remove from each photolabeled protein the carboxy-terminal portion that extends beyond the membrane. Only one, albeit diffuse, band was now observed that on subsequent deglycosylation with endoglycosidase F was more clearly seen to be a single band, indicating that differing lengths of peptide chains were cleaved from the two proteins. These results permit the interpretation that the difference in the two forms of the substance P receptor present in rat submaxillary glands is due to differences in the length of their carboxy termini.