A morphometric analysis of mouse sperm and of their nuclei was undertaken to investigate their respective post-testicular maturation. Sperm were collected from the testis, caput and cauda epididymidis, and their corresponding nuclei were isolated. Results indicate that the post-testicular maturation of sperm is distinct from that of nuclei. The size of intact sperm heads increases in the caput followed by a subsequent decrease in the cauda. In contrast, sperm nuclei decrease progressively in size. In general, a greater magnitude and number of alterations in intact heads and nuclei occur while in transit from the testis to the caput than during passage to the cauda epididymis. These results suggest that the period immediately following their release from the testis is crucial to the complete morphological maturation of sperm heads and nuclei. © 1993 Wiley-Liss, Inc.