The dorsal nucleus of the lateral lemniscus (DLL) in the mouse was studied using the rapid Golgi method. Three types of neurons were observed in the DLL. Type I neurons had a piriform or triangular cell body with a mean diameter of 14 by 19 micron, and emitted 3-5 primary dendrites. The cell bodies of type II neurons were either spindle or piriform in shape and were, on the average, 17 by 26 micron in diameter with 2-4 primary dendrites. Type III neurons had polygonal or triangular cell bodies which were 24 by 31 micron in average diameter and there were 4-6 primary dendrites. The axons of the DLL neurons most frequently traveled medially or ventromedially, and only a few could be followed dorsally among the fibers composing the lateral lemniscus (LL). The afferent fibers of the DLL were separated into three groups: ascending afferents, descending afferents and afferents from the medial aspect. The ascending afferents were collaterals of the LL fibers distributed mainly in the inferior colliculus. The descending afferents were also collaterals arising from the descending LL fibers. The afferents from the medial aspect ran across the tegmental area to distribute in the DLL. In addition, numerous LL fibers gave off terminal collaterals to the DLL. The ascending or descending nature of these LL fibers was not determined. Thus, the DLL is considered to be one of the commissural relay nuclei in the auditory system.