Membranes of boar spermatozoa from different regions of the epididymis and after ejaculation were studied by the freeze-fracture replica technique. The ordered pattern of the intramembranous particles of spermatozoan plasma membranes was different in the five arbitrary zones of the epididymis and in the semen. A distinctive ordered pattern was absent in zone 1, which is the proximal segment of the epididymis. In zone 2, paired parallel rows of the particles were present in the plasma membrane over the acrosomal region. This parallel arrangement was not present in zone 3 spermatozoa. Anterior to the posterior ring, cords formed by packed particles were apparent in zone 2 spermatozoa and reached their maximum prominence in zone 3, and persisted in zones 4 and 5 and in the semen. The plasma membrane over the marginal ridge of the acrosome had a hexagonal array of particles only in zones 4 and 5 spermatozoa. A similar pattern appeared on the post-acrosomal region of spermatozoa in zone 5 and in the semen. The plasma membrane of the middle piece had a rectilinear arrangement of the particles in zone 2 spermatozoa in which the migration of the cytoplasmic droplet was complete. Rudiments of the rectilinear arrangement persisted in spermatozoa in zones 4 and 5 and in the semen. These changes are discussed in relation to sperm maturation in the epididymis. The acrosomal membrane had a hexagonal arrangement of particles in the equatorial segment. The marginal ridge of the outer acrosomal membrane had parallel rows of intramembranous particles. The organization of the acrosomal membrane particles did not change during the epididymal passage of boar spermatozoa.