Postnatal development and differentiation of the rat epididymis was studied in the rat from 15 to 120 days of life using stereological techniques. Both the relative volume (volume density) and absolute volume of the epithelial, interstitial, and luminal compartments in the initial segment, caput, corpus, and cauda epididymides were determined. In all segments the volume density of the epithelial compartment increased between days 15 and 30 before falling to adult values at 45 days in the initial segment (0.476 ± 0.031), at 60 days in the caput (0.258 ± 0.028) and at 90 days in the corpus (0.245 ± 0.007) and cauda (0.140 ± 0.004). The relative volume of the interstitium decreased, whilst that of the lumen increased over the same period with adult values being achieved earlier in the proximal segments than in the distal segments. In contrast to volume fraction the absolute volume of all compartments in all segments increased from day 15 to day 90. Between 90 and 120 days the absolute volumes of compartments in the initial segment and caput showed little volume change. All compartments in the corpus and cauda showed significant increases in volume over the same period. A similar pattern of development was observed with respect to the surface area of both the luminal and basement membrane aspects of the epithelium; surface area per unit volume (surface density) in all segments reached adult values at approximately 60 days, whilst the increase in absolute area of the surfaces ceased at 90 days in the initial segment and caput and continued to 120 days in the corpus and cauda. The total length of the epididymal tubule showed the same pattern with no increase in length apparent in the initial segment and caput after 90 days. Length however continued to increase in the corpus and cauda between 90 and 120 days. Tubule and luminal diameter reached their definitive values in the initial segment on day 60 but continued to increase until day 90 in the distal segments. Epithelial height increased between 15 and 30 days in all segments; in the caput, epithelial height was greatest on day 30 before decreasing slightly; and in the corpus and cauda maximal epithelial height was observed on day 45 decreasing to stable values on day 60 and 90, respectively. All the data is consistent with a proximal to distal differentiation and development of the epididymis with growth continuing in all segments after the establishment of the definitive architecture. Growth of the distal segments continued after its cessation in the proximal segments. © 1994 Wiley‐Liss, Inc.