Abstract The composition of the bronchial branches of the vagus nerves was studied in cats using light and electron microscopy. In order to determine the number and the diameter of fibers in the afferent and the efferent components, a unilateral efferent vagotomy was performed. The myelinated and the non-myelinated fibers were counted from the total nerve area and the endoneural area of each one was measured by means of a computer. Composition of “afferent” bronchial nerves (after degeneration following the efferent vagotomy) was compared to that of “entire” nerves. The main results are: (1) efferent fibers represent about 40% of the “entire” bronchial nerve; (2) non-myelinated fibers constitute more than 90% of the total population of the “entire” nerves as well as of the afferent component: (3) the density of myelinated and non-myelinated fibers (i.e. their number per surface unit) was similar in all nerves. However, there were discrepancies between diameter histograms established from different areas of a section. This feature seems to be due to preferential compartmentalization by Schwann cell envelopment of fibers having comparable diameter. This “packing effect” was observed in both efferent and afferent components.