Male and female hamsters were fed natural ingredient diets containing 6, 12, 18, or 24% protein. Differences in growth rate, feed consumption, kidney weight, serum chemistry (total protein, serum creatinine, blood urea nitrogen), and longevity were analyzed. The kidneys were examined microscopically and graded according to the severity of nephritis. The 6% protein diet resulted in the lowest frequency of nephritis, but the animals were significantly smaller and had shorter life spans than the remaining groups. The 12% protein diet initially promoted slower growth than 18 or 24% protein, but the differences in body weight eventually became insignificant. The 12% protein diet resulted in a higher frequency of nephritis than 6% protein, but lower than 18 or 24% protein. The 18 and 24% protein diets promoted the most rapid early growth as well as the highest frequency and most severe kidney lesions. Uremia did not appear to be a major cause of death. Longevity did not differ significantly among hamsters of either sex fed 12, 18, or 24% protein diets.