The ultrastructure of the mouse sweat gland was examined, in support of neurological studies of sweat glands and their relationships to the autonomic nervous system. It was found that the mouse sweat gland is similar to that of the rat and has only one type of secretory cell. Many nerve fibers are entwined with the secretory tubule and contain accumulations of round, clear vesicles, some microtubules, but apparently no neurofilaments. Cholinesterase is found in the clefts between nerve fibers and their ensheathing Schwann cells. The nerve fibers tend to run parallel with capillaries, but have no close association with either the capillaries or the secretory epithelium. Capillaries provide an abundant blood supply to the sweat gland and are fenestrated. The relationships between cellular elements of the sweat gland provide no direct evidence of the mechanisms involved in neurogenic sweating, although it seems likely that effector substances are diffusely distributed.