The author studies the effect of ethionine upon the acrosome formation of spermatozoa in albino rats. Five tenth percent of DL-ethionine was added to the diet of the experimental group rats and they were sacrificed after l,2,3, and 4 weeks, respectively. The testes were observed by electron microscope: The results obtained were summarized as follow: The changes were decrease in electron density and irregularity of margin in acrosomes, vacuolization, irregularity of margin and increase or decrease in electron density in head cap, poorly defined border between acrosome and head cap, and decrease of Golgi remnant. These changes were occurred at second to third weeks from start of ethionine feeding and more severe at fourth week. These overvation suggested ethionine induced degenerative changes of acrosome and its related structures, characterized by decrease of electron density, vaculization and irregularity of their margin. The degenerative changes of these structure were considered to be induced by secondary effects of damage to other organells, such as mitochondria and so.