To examine the effects of excessive animal protein intake on the urinary risk factors of calcium oxalate stone formation, protein loading experiments were performed on oxalate stone formers and normal subjects and rats. By animal protein loading of 61 g/day with diet which contained 400 mg of calcium and 46.6 mg of total oxalate, urinary calcium excretion increased by 27% both in stone formers and in normal subjects. Urinary oxalate excretion had a tendency to decrease on the 3rd and 4th days after loading. Urinary uric acid excretion increased significantly after loading. No significant change was recognized in the acid glycosaminoglycan (AGAG) and magnesium excretion. Urinary calcium excretion increased both in the high and low calcium diet groups after animal protein loading in the rat experiment. Urinary oxalate excretion increased after loading only in high oxalate low calcium diet group, therefore, it was supposed that animal protein intake caused increase of intestinal oxalate absorption.