1. Afferent fibers supplying the horizontal and superior semicircular canals of the chinchilla were labeled by extracellular injections of horseradish peroxidase (HRP) into the vestibular nerve. The arborizations of labeled fibers within the sensory epithelium were reconstructed from serial sections of the crista. 2. The sensory epithelium of the crista can be divided into central, intermediate, and peripheral zones of approximately equal areas. The three zones can be distinguished in normal material by the density of hair cells and by the morphology of calyx endings. 3. Labeled fibers supply either the canalicular or the utricular side of the crista. Axons seldom bifurcate below the basement membrane and they begin dividing into their terminal arborizations almost immediately upon entering the sensory epithelium. The arborizations are compact, seldom extending more than 50 micron from the parent axon. 4. Both calyx and bouton endings were labeled. Calyces can be simple or complex. Simple calyces innervate individual hair cells, whereas complex calyces supply two to three adjacent hair cells. Complex calyces are commonly found only in the central zone. Simple calyces and boutons are located in all regions of the epithelium. Calyces emerge from the parent axon or one of its thick branches. Boutons, whether en passant or terminal, are always located on thin processes. 5. Fibers were classified as calyx, bouton, or dimorphic. The first type only has calyx endings, the second only has bouton endings, and the third has both kinds of endings. Dimorphic units make up some 70% of the labeled fibers, bouton units some 20%, and calyx units some 10%. The three fiber types differ in the diameters of their parent axons and in the regions of the crista they supply. Axon diameters are largest for calyx units and smallest for bouton units. Calyx units are concentrated in the central zone of the crista, whereas bouton units are largely confined to the peripheral zone. Dimorphic units are seen throughout the sensory epithelium. 6. Calyx units are almost always unbranched and end as simple calyces or, less often, as complex calyces. The terminal arbors of bouton units consist of fine processes containing 15-80 endings. Dimorphic units vary in complexity from fibers with a single calyx and a few boutons to those with one to four calyces and more than 50 boutons. 7. The results emphasize the importance of dimorphic units, which were the most numerous type of afferent fiber labeled in this study and were the only units found to innervate all regions of the sensory epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)