The objective of this study is to increase our understanding of the regulation of individual nephron glomerular filtration rates (SNGFR) by exploiting the unique structural arrangement of the avian kidney. This kidney has a population of nephrons resembling reptilian nephrons which function independently of each other and do not contribute directly to the concentrating mechanism. These can cease filtering altogether under some circumstances. The avian kidney also has a population of nephrons resembling mammalian nephrons which function together in the concentrating mechanism. These nephrons do not normally cease filtering, but their filtration rates can change under some circumstances. We are interested in three major aspects of this broad objective: 1) the effects of factors such as antidiuretic hormone and sodium chloride on the regulation of SNGFRs within the different nephron populations; 2) the intrarenal blood flow patterns and their regulation by factors such as antidiuretic hormone; and, 3) the effects of alterations in SNGFR on the renal concentrating mechanism. SNGFR will be measured by de Rouffignac, Diess, and Bonvalet's (1970) modification of Hanssen's (1963) sodium ferrocyanide technique. The intrarenal patterns of blood flow distribution will be studied by injection of the renal vasculature with a silicone elastomer in order to determine possible glomerular by-pass pathways, the vascular sites of action of control factors, and the relationship of the renal microvasculature to the nephrons. Medullary cone electrolyte gradients will also be analyzed and related to the SNGFRs of nephrons contributing to those medullary cones.