Abstract Age-related morphologic properties of muscle cells and of their innervation in the gill were sought to explain reduced contractility in old Aplysia. As a consequence of previous physiological findings, properties of two muscle groups were examined: one group, MPn, innervated by motor neuron, L 7, whose ability to elicit muscle contraction was reduced during aging, and the other group, LEV, innervated by motor neuron, LDG 1, whose ability was not. In neither group did muscle properties, such as cell diameter and density and thick filament diameter and density, and resting potential, change with age. In contrast, age-related remodeling of nmjs did occur. The results show that remodeling is expressed differently in the two types of junctions: In L 7-nmjs contact between terminals and muscle cells significantly increased with age; in LDG 1-nmjs the terminal perimeter was enlarged significantly and not the contact between terminals and muscle cells. With increased age, the proportion of the terminal perimeter in contact with muscle in L 7-nmjs increased significantly, and in LDG 1-nmjs it remained essentially the same. Accompanying the remodeling of LDG 1 terminals was a significant increase of vesicles in them; no significant change in vesicle number was measured in L 7 terminals. The effect on transmission of the age-related presynaptic changes in the two types of junctions is discussed.