Cochlear innervation patterns were studied in infant cats and rats with the rapid Golgi method. Examination of thick serial sections and surface preparations with the differential interference contrast microscope (Nomarski optics) allowed direct visualization of individually impregnated spiral ganglion cells, complete with their peripheral processes and endings in the organ of Corti. Individually impregnated efferent fibers could be recognized as heavily varicose axons that project radially to endings beneath inner and outer hair cells after taking a tangential course in the intraganglionic spiral bundle. It was often possible to visualize unimpregnated hair cells in contact with the impregnated endings of both types of fibers. There are at least two types of spiral ganglion cells in the cochlea of the infant cat and rat. One type innervates only inner hair cells by means of radial fibers. These ganglion cells constitute the overwhelming majority of ganglion cells impregnated in our preparations, and each cell typically innervates two inner hair cells. Hence, these ganglion cells establish nearly "point-to-point" connections between the auditory nerve and the organ of Corti. The other type of ganglion cell innervates outer hair cells by means of long spiral fibers; each cell typically innervates many outer hair cells through the numerous angular enlargements and short end branches of its spiral fiber. In addition, a few of these spiral fibers also send branches to inner hair cells by means of short collaterals; it remains to be seen if such fibers also occur in mature cochleas. Efferent fibers have been traced to inner and outer hair cell regions. The simplest pattern is formed by fine beaded axons with only a few branches ending mainly beneath inner hair cells. More complex patterns are formed by larger axons with many branches ending beneath inner or outer hair cells. Many efferent fibers send branches to both inner and outer hair cells. Electrophysiological studies so far have not demonstrated different populations of units that clearly correspond to the spiral and radial fibers. Therefore, the physilogical differences between inner and outer hair cell innervation remain undefined.