Soay rams of the semidomesticated sheep were used in this experiment. They were given a s.c. implants containing melatonin and after 11 weeks the rams were killed. In contrast to the control group of animals the melatonin-treated rams showed histomorphological and morphometric al changes as a result of reactivation of the glandular tissues of the seminal gland. Particularly, the secretory principal cells were increased in number and height and showed signs of increased seceretory activity; apical cytoplasmic protrutions became well-developed and covering the inner surface of the glandular end-pieces of the seminal vesicle. The basal cells were displayed mitotic divisions. The glandular portions were increased in size and the interstitial connective tissues were reduced. The overall results are consistent with the view that melatonin can be used commercially to modify the timing of th breeding season in sheep.