The pattern of protein synthesis was studied in the pancreatic beta cells of the Egyptian sand rat (Psammomys obesus). When fed a standard Purina Laboratory Chow diet instead of a leafy vegetable diet, these animals develop the characteristic signs of diabetes mellitus. Tritiated leucine was injected intravenously into pairs of sand rats (one on a vegetable diet and one on a Purina Laboratory Chow diet). Two pairs of animals were sacrificed at 5-, 20- and 60-minute intervals, and pancreatic tissue was studied by electron microscopic autoradiography. At 5 minutes, the relative grain density was greatest over the rough endoplasmic reticulum; at 20 minutes it was greatest over the Golgi complex and at 60 minutes, over the granules. There were no statistically significant differences in the relative grain densities over the rough endoplasmic reticulum, over the Golgi complex or over the secretion granules between the sand rats on the vegetable diet and Chow diet. These results show that in the early phase of the development of diabetes mellitus, the pattern of protein synthesis in the beta cells of the normal and diabetic sand rat compares with that of other endocrine glands. The tritiated leucine was apparently incorporated into the newly synthesized secretory product in the rough endoplasmic reticulum during the first 5 minutes. The formed product migrated to the Golgi complex at 20 minutes, and at 1 hour was seen mainly over the light granules. In addition, there was no obvious difference in this pattern of protein synthesis between the normal and diabetic sand rats. This suggests that the secretory product, considered to be mainly insulin, is produced in the usual or in increased amounts, but it is not fully utilized by the diabetic animal and remains in circulation, thus increasing the plasma insulin level.