The development of cell types and regional differences in the rat epididymis was studied in specimens of the initial, middle and terminal segments prepared at intervals between birth and postnatal day 94. The development of the epididymis was divided into three phases: (1) an undifferentiated period; (2) a period of differentiation, and (3) a phase of expansion. During the undifferentiated period, from birth to day 15, the epithelial cells had a uniform appearance. Halo cells, which are believed to be migratory leukocytes, appeared on day 14. The period of differentiation extended from day 16 to day 44. Slender, densely staining cells, termed narrow cells, appeared in the epithelium of all three segments on day 16, constituting the first evidence of differentiation of cell types in the epididymal epithelium per se. In addition to their shape and apical nuclei, the narrow cells were distinguished from other epithelial cells by the presence of cup-shaped apical vacuoles and mitochondria with tubular cristae. Principal cells and basal cells were identified on day 28, which also marked the firsh distinction of differences in epithelial height among the different segments. Narrow cells persisted into the adult in the initial segment. In the middle and terminal segments, however, narrow cells disappeared by day 35, when light cells made their appearance. The major event of the period of expansion, from day 45 to 3 months, was the appearance of sperm in the lumen between days 45 and 52. A model for differentation of cell types in the epididymis is proposed and it is suggested that narrow cells are precursors to light cells in the middle and terminal segments. The development of ultrastructural features of adult cell types preceded the appearance of sperm in the lumen.