Dietary protein restriction is known to be beneficial in the preservation of renal function when renal mass is reduced. This study investigates the effects of two different dietary proteins, casein and soya, upon renal function in normal rats and rats subjected to subtotal nephrectomy. The diets were isocaloric, with identical sodium, potassium and phosphorus contents. Normal rats ingesting a 24% soya protein diet demonstrate lower effective renal plasma flow rates and lower glomerular filtration rates than rats ingesting a 24% casein diet. Experimental animals were subjected to a unilateral nephrectomy and contralateral partial renal infarction and were fed either casein or soya, at 24 or 12% levels, for 3 months. Those animals ingesting the soya diets demonstrated improved survival (p < 0.05), less proteinuria (p < 0.02), less renal hypertrophy (p < 0.005) and less renal histological damage. The nature of the dietary protein appears to influence both normal renal function and the progression of experimentally induced renal disease.