Abstract: The present study aims to pinpoint the effects of the flavour enhancer Monosodium Glutamate; MSG on the histology and ultrastructure of the testes. Such effects were monitored after giving 10 (Short-term) and 30 (Long-term) oral doses each was 4 g kg^-1 body weight; a third of LD₅₀, of MSG to adult white rats Rattus norvegicus. Their testes were routinely processed for examination with the light and electron microscopes. The seminiferous tubules of rats treated with MSG for short-term exhibited slight to moderate damage. The slight damage of the tubules included appearance of vacuoles and loss of late spermatids. The vacuoles were found inside the cytoplasm of the spermatogonia, the primary spermatocytes and Sertoli cells. Some seminiferous tubules displayed sloughing of impaired early spermatids into their lumina. Other tubules showed complete loss of early and late spermatids. Due to shrinkage of many tubules the intertubular spaces widened. By using the electron microscope, the primary spermatocytes revealed loss of cristae of many of their mitochondria, great enlargement of Golgi apparatus and disruption of the nuclear envelope as well. The sloughed early spermatids possessed irregular head caps, dilated endoplasmic reticulum, mitochondria with fused cristae and few cytoplasmic vacuoles. Also, few mitochondria of these cells underwent disorganization of their distribution. Mitochondria of Sertoli cells exhibited loss of some or all of the cristae. In case of long-term treatment with MSG, the testes of rats contained severely damaged tubules. Such seminiferous tubules were characterized by containing only Sertoli cells almost arranged in a ring together with occasional occurrence of ruined spermatogenic cells and appearance of masses of necrotic germ cells. Many mitochondria with disintegrated cristae were noticed in the severely perished primary spermatocytes. Sertoli cells showed extensive vacuolation, disintegration of the cytoplasm and highly harmed mitochondria with electron-dense matrices. The structural changes of testes of the MSG-treated rats were found to be dosage-duration-dependent and ranged from slight to moderate damage in case of the short-term treatment however, severe damage was recorded in the case of the long-term treatment. Thus, it is important to reconsider the usage of MSG as a flavour enhancer.