An analysis of glomerulo-tubular balance in the rat proximal tubule. Flow dependence of absolute proximal reabsorption (APR) or glomerulo-tubular balance (GTB) has been observed with spontaneous alterations in flow and attributed to both intraluminal and extraluminal factors. Flow dependent alterations in APR were demonstrated when 1. nephron filtration rate (SNGFR) was decreased by tubulo-glomerular feedback mechanisms by increasing late proximal tubular microperfusion rates, and 2. when SNGFR was increased by addition of [Sar1, Ala8] angiotensin II to the adjacent peritubular capillary flow. Selective reduction in early proximal tubular flow rate by pump aspiration also resulted in flow dependent reductions in APR. However, selective additions of perfusion fluids of various native and artificial constituency to the early proximal tubule did not result in flow dependent increase in APR. Conclusions. 1. GTB with both increases and decreases in SNGFR can be demonstrated at the level of the single nephron, 2. selective reductions in luminal flow rate produces parallel reductions in APR; however, 3. increases in flow rate with either artificial or native fluids of different ionic concentrations did not result in increases in APR. This lack GTB may be due to lack of parallel changes in peritubular physical factors or that APR in the S2 segment is less sensitive to increase in flow rate.