The blood serum fasting levels of inorganic sulfate increase with age, other differences are largely due to diet. The serum sulfate shows a circadian rhythm with a minimum before noon and a maximum in the afternoon and evening. Urinary excretion of sulfate disappears during a low-protein diet. The serum sulfate levels rise temporarily if oral intake of protein, water or inorganic sulfate in non-laxative doses is increased. From the decomposition of protein, sulfate is first excreted with alkali cations and sooner than other catabolites. Drinking water mobilizes sulfate from its tissue pool, which can be blocked by the addition of sodium chloride. The subsequent excretion of an oral sulfate dose is prolonged over one day.