Three physiologically characterized spindle (group Ia) afferents were labeled by the intracellular injection of HRP and were processed for light-level reconstruction. Thirty-five boutons in the ventral horn were then selected for analysis. They were serially thin sectioned and characterized in terms of volume, total surface area and the surface area of apposition to postsynaptic neurons (apposed surface area), mitochondrial volume, vesicle and active zone features, relation to presynaptic contacts, postsynaptic profile size, and position within the terminal arbor. Virtually all of these characteristics were widely variable, both within the entire population and in the endings of a single fiber. Apposed surface area, mitochondrial volume, vesicle number, active zone vesicle number, active zone number, and total active zone area were highly correlated in a positive linear manner with bouton volume. This suggests a type of ultrastructural “size principle,” in which the morphological features associated with synaptic release scale directly in proportion to bouton size. This pattern also extends to local circuit interactions: the extent of an Ia bouton's input from axoaxonal contacts (86% receive at least one axoaxonal contact) was directly proportional to its size. In addition, the characteristics of an Ia bouton were related to its position on the postsynaptic element and within the terminal arbor. Vesicle density, percentage mitochondrial volume, and active zone size increased as the postsynaptic process decreased in size, while volume, apposed surface area, active zone number and area, and vesicle number all decreased as one moved downstream within a terminal branch, with the exception of the terminal bouton. Vesicle density also decreased as one moved away from the dorsal root entry zone.