Radioautographic studies were conducted on early developmental stages of the inner ear in monkey (Macaca mulatta) embryos ranging in age from stage 11 to stage 20 (25 to 39 days' gestation) and labeled for one hour with [3H] thymidine. The most active period of proliferation occurred at stage 13, at which time all regions of the otocyst, including the endolymphatic duct, were heavily labeled. In subsequent stages the dorsal portion of the endolymphatic duct failed to incorporate label, whereas proliferation continued in the ventral portion of the duct near its communication with the medial wall of the otocyst. The duct thus appeared to grow upward as a result of cell division from below. Mitotic activity continued in the remainder of the otocyst although there was a progressive temporal decrease in the labeling index. Spatial gradients also occurred, with dorsal and lateral regions less active than ventral and medial ones. As the otocyst differentiated, the cells in various areas became attenuated. These flattened cells were still capable of undergoing cell division, as indicated by the presence of labeled nuclei in all regions of the utricle, saccule, semicircular ducts, and cochlear duct.