Electron microscope autoradiographic studies of RNA synthesis in Sertoli cells of the adult mouse, using [3H]uridine as a precursor, show a random distribution of grains in the nucleoplasmic area and a localized product on the nucleolar fibrillar component at early labeling times (15 min). The condensed chromatin bodies associated with the nucleolus seem to have a nucleolar organizing function. At later labeling times (3 h) certain interchromatin granules occurring in clusters in the nucleoplasm become partially labeled. After 7-8 days, the disappearance of label from Sertoli cells, contrasting with the longer persistence in spermatocytes, suggests a rapid turnover of RNA in Sertoli cells. Differential labeling patterns and structural changes in Sertoli cells cytoplasm are related to variations in functional activities presumably required for normal spermatogenesis and/or spermiogenesis.