Male Donryu rats were housed in a specific pathogen free room and fed with 60 % of the mean daily quantity of diet consumed by rats fed ad libitum. The animals were divided into three groups of 50 and each group fed different amounts and types of dietary protein. The group P40 was fed with a diet containing 40 % of total volume as vegetable protein; the agroup P10 with 10 % of total volume as vegetable protein; and the group CE7 with 18 % of total volume as mixed vegetable and animal proteins. Subsets of the animals were sacrificed at the ages of 6, 12, 18, 24 and 30 months. The P40 group gained body weight faster than other two groups during the first months. However, after 12 months. the body weight began to decrease, reaching the lowest mean body weight among the three groups by the end of the study. The three groups did not show any significative difference in the muscle fiber number index and in the number of capillaries per muscle fiber. The calculated diaphragm volume index increased up to a peak at 12 months in all three groups decreasing gradually thereafter, but did not show any significant difference due to dietary protein intake. The data suggest that the morphological age changes occurring in the diaphragm muscle are likely unrelated to dietary conditions and are more likely due to other factors such as changes in the neural activity during aging.