As renal mass is lost, adaptive changes occur within the residual tissue which aid in the maintenance of external K balance. The collecting system is the nephron site where these adaptive changes occur. These conclusions were based on observations made in rats in which the superficial distal tubule was not a source of K addition to the urine. The effect of a reduction in renal mass on K transport in the superficial distal tubule of rats in which this site is a potential source of urinary K was studied. At 10-14 days after sham operation or partial nephrectomy, the glomerular filtration rate (GFR) (ml/min) in sham-operated control rats (stage I), 2.40 .+-. 0.26 (SE), was comparable to the value of 2.37 .+-. 0.23 after 25% nephrectomy (stage II). Both values were significantly greater than the GFR of rats with 75% nephrectomy, 1.25 .+-. 0.12 (stage III). Absolute rates of K excretion (.mu.eq/min) were similar: stage I, 1.52 .+-. 0.22; stage II, 1.71 .+-. 0.02; stage III, 1.69 .+-. 0.20. Absolute delivery of K to the late distal tubule of stage III rats, 127 .+-. 15.2 peq [pico equivalent]/min, was greater than that of stage I rats, 39 .+-. 4.4 (P < 0.005) as a result of both an increase in K delivery to the early distal tubule, 13 .+-. 2.5 vs. 46 .+-. 10.7 (P < 0.02) and K secretion along the distal tubule, 26 .+-. 3.9 vs. 82 .+-. 14.6 (P < 0.01). The rats with 25% nephrectomy (stage II) were not a homogeneous group and the quantity of K secreted in the distal tubule, 42 .+-. 11.8 peq/min, was not different from stage I or III rats. The mean fractional excretion of K in the urine (U) was not different from the mean fractional delivery of K to the late distal (LD) tubule: stage I LD, 20.7 .+-. 1.4 vs. (U) 16.1 .+-. 1.8%; stage II LD, 23.5 .+-. 4.2 vs. (U) 26.1 .+-. 4.3%; stage III LD, 30.8 .+-. 3.5 vs. (U) 31.7 .+-. 2.7%. The relationship between the parameters was such that both net addition and net reabsorption of K was observed beyond the distal tubule in individual experiments. The distal tubule may contribute to the maintenance of K balance in the presence of renal insufficiency, and other factors, e.g., an increase in delivery to the distal tubule, and, on occasion, addition of K beyond this site, may also be of importance.