The isolated perfused kidney of the rat will be used as a model for the investigation of factors regulating renal function. Special attention will be given to the endocrine functions of the kidney. Kidneys from normal and phosphate-deprived rats will be perfused with radiolabelled 25-OH-cholecalciferol to study the process controlling the renal activation of vitamin D. Renin secretion by the perfused kidney and its relationship to the macula densa will be studied in two non-filtering models -- production and factors influencing it will also be studied. The relationship between oxygen consumption, substrate availability, and transport work by the perfused kidney will be investigated. The active transport of chloride will be studied in isolated tubes and cells from a variety of tissues and its dependence on sodium transport and the activity of Na-K-ATPase will be investigated.