KOKUBO, T., TAKAHASHI, M., FURUKAWA, F., NAGANO, K., HAYASHI, Y. and TAKAHASHI, T. A Comparative Histometrical Analysis of Renal Inner Medulla of Man, Swine, Dog and Hamster. Tohoku J. exp. Med., 1984, 142 (1), 77-88-Quantitative estimations of the total circumference (in mm) of Henle's thin segment (TCH) and of the collecting tubules (TCC) in the renal inner medulla of man, swine, dog and hamster were made at different distances (X) from the papillary apex. The results were expressed as exponential functions of X, the distance in mm along the medullary ray from the papillary apex to a certain point of the inner medulla, as L=Ae^BX In man and swine, TCH was greater than TCC towards the external border of the inner medulla. However, in dog and hamster TCH was less than TCC along the entire inner medulla. The exponents of TCH showed nearly constant values (hamster: 0.68, dog: 0.75, man: 0.55, swine: 0.56) but those of TCC had values (hamster: 0.92, dog: 0.57, man: 0.28, swine: 0.26) correlated with the ability for concentration of urine. These findings indicate a good correlation between the ability to concentrate urine and the structure of the collecting tubules of the inner medulla, suggesting that the structure of the collecting tubules in the inner medulla affects the mechanism of urinary concentration. urinary concentration ability ; total circumference of collecting tubules ; exponential function