The purpose is to define the mechanisms of bicarbonate transport in individual isolated perfused nephron segments of rabbit kidneys. We are measuring bicarbonate transport in proximal straight and cortical collecting tubules and determining the dependence on sodium, chloride and carbonic anhydrase. BIBLIOGRAPHIC REFERENCES: Burg, M., Patlak, C., Green, N. and Villey, D.: Organic solutes in fluid absorption by renal proximal convoluted tubules. Am. J. Physiol. 231:627-637, 1976. Warnock, D., Burg, M. and Orloff, J.: Urinary acidification: Co2 transport by the rabbit proximal straight tubule. Am. J. Physiol. 1:F20-25, 1977.