Background: Monosodium glutamate (MSG) is one of the most flavor enhancer commonly used all over the world by millions of people in their food. It is considered a toxic substance affecting many organs in the body. Its effects on male gonads include impairment of endocrine function of the testis as well as the process of spermatogenesis with significant oligozoospermia and increased abnormal sperm morphology. Objectives: This work is performed to throw more light on the harmful effects of food additives containing MSG on the structure of the testis of adult albino rats. Materials and Methods: Sixteen adult male albino rats with average weight of 185 g were randomly classified into two groups (A, B), eight rats in each group. Group A served as control group received equal amount of feeds (growers' mash), while group B was the treated group, were given 3gm of monosodium glutamate, mixed with the growers' mash. for fourteen days. The rats were sacrificed on the fifteenth day of the experiment. The testis was quickly dissected out and fixed in 10% formalin for light microscopic study and in glutaraldehyde for electron microscopic study. Results: Light microscopic examination, showed evidence of cystic degenerative changes, presence of intercellular vacuolations in the stroma of the testis when compared to the control sections. There were atrophic and degenerative changes and widening of the intertubular space. With electron microscopy, the majority of the seminiferous tubules showed loss of the late spermatids. Vacuoles were noticed close to the periphery of the tubules. Mitochondria of Sertoli cells and the primary spermatocytes exhibited different degrees of damage such as loss of some or all their cristae. Leydig cells showed few lipid droplets with multiple degenerated mitochondria and faint smooth endoplasmic reticulum. Conclusion: MSG affects the reproductive function of male albino rats through its toxic effect on the testis. Key words: Monosodium glutamate - (MSG)- Testis .