In rats fed with various diets as regard contents of magnesium, cholesterin, neutral fat, carbohydrates and proteins, urinary thyroxine (U-T4) appears low in general when compared with control chow, whereas renal calcifications and stones are absent only with the latter and the carbohydrate-rich diet. Normalization of U-T4 is hampered by differences in urinary creatinine excretion and in individual weight gain. In male renal calcium stone patients (20--40 years) U-T4 also is lower than in age-matched controls, but in other groups no abnormality could be detected.