Marked atrophy of the mucosa develops in self-emptying blind loops of rat jejunum. The application of microdissection techniques revealed a 74% reduction in the mucosal surface area of the blind loops, 3 weeks after their creation. Kinetic studies of the influx of L-phenylalanine and beta-methyl-D-glucoside into rings of blind loops showed that the Km for both mechanisms was unchanged, but that there was a small reduction in the Vmax for both substrates, when expressed in terms of unit weight of incubated tissue; this attained 31% for phenylalanine and 12% for beta-methyl-glucoside. When the kinetic data were combined with the morphometric results, it could be shown that the Vmax for both substrates, when expressed per unit area of mucosal surface, was more than twice as large in the blind loop epithelium as in normal tissue. Such findings indicate either that the individual cells of the blind loop epithelium possess more transport sites than normal, or that there is a greater proportion of absorptive cells within the blind loop mucosa.