The fine structure and composition of the nerve plexuses associated with the cervical and the thoracic parts of the trachealis muscle were studied in two groups of six guinea-pigs. One group of animals was perfused with fixative before removal of the specimens. In the second, the specimens were incubated in 5-hydroxydopamine before cold immersion fixation. In both the cervical and the thoracic trachea, plexuses of nerves were found within and between the fascicles of muscle cells. The number of intrafascicular nerves/1000 micrometer 2 was low, and only a small proportion of the nerves contained more than five axons. Interfascicular nerves were more numerous and the mean number of axons/nerve was significantly higher than within the muscle. The number of efferent terminal profiles/100 axonal profiles was higher in the intrafascicular than in the interfascicular plexus, but in neither case was there evidence of a very close approximation of exposed terminal membranes to the membranes of adjacent muscle cells. Terminals classified as those of autonomic efferent axons were present in much greater numbers in the nerves than any other type of terminal. Examination of 5-hydroxydopamine-incubated specimens showed that less than half of these terminals were the terminals of adrenergic axons. Terminals of the type considered to represent the terminals of purinergic axons were also found in the nerves but were present in much smaller numbers than in the nerves of the submucous plexuses. The presence in the nerves of mitochondria-containing terminals with features similar to those of mechanosensitive nerve endings was related to physiological evidence of the location of stretch receptors in the muscle.