The three-dimensional branching pattern and ultrastructure of afferent myelinated fibers and their terminals located in the trachealis muscle of the dog are described. The afferent endings are believed to be those of the slowly adapting stretch receptors of the trachea. They have structural features typical of mechanoreceptors: distal to the loss of myelin, their shape becomes more irregular and the cytoplasm is filled with mitochondria, glycogen, and osmiophilic bodies. In some places the cell membrane is attached directly to basal lamina without interposition of a Schwann cell. A bundle of unmyelinated fibers accompanies each myelinated fiber and continues for an undetermined distance beyond (luminal to) terminations of the myelinated fiber. The unmyelinated fibers contain many round, clear vesicles and a few dense-cored vesicles and are also attached directly to basal lamina in places. Three-dimensional reconstruction of three receptors revealed three quite different branching patterns, but all included apparent rings as part of more or less contorted terminal regions (some neurons apparently having more than one terminal region). No obvious structural basis for the activation of receptors by transverse but not longitudinal stretch was found.