The mechanism of glomerulotubular balance was investigated in normal rats expanded with isotonic saline and in uninephrectomized rats, both nondiuretic and expanded. Micropuncture collections were made from proximal tubules during free flow, followed by re-collections from the same sites during reduced GFR produced by partial occlusion of the renal artery or aorta. In other tubules collection was made first during reduced GFR, followed by re-collection after release of arterial compression. Transit times through proximal tubules and loops of Henle were measured with lissamine green. Similar results were obtained in all groups of rats. GFR was reduced an average of 45% by clamping. Proximal inulin F/P ratios were slightly but significantly elevated, and transit times (TT) prolonged, during reduced GFR. Velocity of tubular flow (1/TT) varied linearly with GFR. Tubular radius was unchanged during alteration in GFR, whether measured directly in photographs of the kidney surface or calculated from nephron filtration rates, TT, and inulin F/P ratios. Reabsorptive rate per unit proximal tubular volume (C/πr2) was invariably decreased with decreased GFR under steady-state conditions, but C/πr2 changed unpredictably immediately after release of the arterial clamp in the non-steady-state condition. Filtration fraction and PAH extraction were unchanged during clamping, and changes in GFR were accompanied by proportional changes in renal plasma flow. These results exclude tubular volume as a primary determinant of the proximal reabsorptive rate but are consistent with the hypothesis that colloid osmotic pressure may play a role in the control of tubular reabsorption. Alternatively, the rate-limiting factor may be the availability of energy for sodium transport, which in turn is related to renal plasma flow, et cetera.