A one-nephron model has been extended to include both short-looped and long-looped nephrons. Variables are volume flow, Na+, K+, Cl-, urea, hydrostatic pressure, and electric potential. The ratio of short-to-long-looped nephrons, one of the parameters of the model, is 5 to 1. With either rabbit or hamster permeability data from perfusion experiments, the model develops an osmolality of approximately 600 mosmol/l at the junction of inner and outer medulla but no osmolality gradient in the inner medulla. With the rabbit data, osmolalities in excess of 1,000 mosmol/l can be generated in the papilla with no active transport if urea permeabilities are less than 10(-5) cm/s; with the hamster data, electrolyte permeabilities must also be reduced. With these modified parameters, urea concentrations are less in the long loops than has been found on micropuncture. These can be increased to experimental levels by increasing the urea permeability and decreasing the hydraulic permeability of thin descending limbs in the inner half of the inner medulla, but to maintain loop osmolality at 1,000 mosmol/l it is necessary to postulate active NaCl transport in thin ascending limbs in the outer half of the inner medulla. This gives an alternative mode of concentration without active transport in the inner half of the inner medulla, in which electrolytes diffuse out of and urea diffuses into both limbs of Henle's loop and mix in the core with urea and water entering from the collecting duct. Concentration in either mode requires significant modification of perfusion data.