Vitamin A deficiency was induced in young adult male rats by feeding them with a synthetic vitamin-A-deficient diet. The histology of their testes was examined periodically from 1 to 20 days following the point where the animals ceased to gain weight. The earliest sign of histological change in the seminiferious tubules, which occurred at day 3, was the sloughing of spermatids and spermatocytes into the lumina of the seminiferous tubules. Pyknotic cells were seen throughout the epithelium in a significant number beginning at day 4 or 5. Most of them were dead spermatocytes and early spermatids. Abnormal multinucleate giant cells appeared during days 6 and 8, and were thought to be abnormal spermatids which failed to complete cytoplasmic division and further differentiation into spermatozoa; eventually they degenerated. Quantitation of germinal cells in the tubules revealed that the number of spermatids decreased sharply between days 2 and 8, and reached zero level at day 10 after the animals ceased to grow. The number of spermatocytes decreased rapidly between days 5 and 12, and the rate of their decrease was relatively slow during the periods preceding and following this interval. Few spermatocytes remained in the tubules at day 20. In comparison, the decrease of spermatogonia was more gradual. At the end of the experiment, the number of spermatogonia in each tubule was about one fourth of that detected at the time when the animals ceased to grow. It was thought that the sloughing and the disintegration of the spermatocytes and spermatids were major factors responsible for their rapid disappearance from the tubules, and that fast dividing and differentiating cells like spermatocytes and spermatids were more vulnerable to vitamin A deficiency than spermatogonia.