Experiments were performed on chronically salt-loaded rats to determine whether resetting of tubuloglomerular feedback is caused by changes in the sensitivity of the juxtaglomerular apparatus itself or by changes of tubular fluid composition. The feedback response was quantified in both salt-loaded and salt-deplete rats by measuring early proximal flow rate (EPF) during loop perfusion at 40, 10, and 0 nl/min using tubular fluid harvested from both groups and with Ringer solution. In salt-loaded rats endogenous tubular fluid produced only a small feedback response (EPF40-0 = 1.9 +/- 1.5 nl/min), whereas exogenous tubular fluid from salt-deplete rats or Ringer solution produced normal feedback responses (EPF40-0 = 15.4 +/- 2.0 and 10.6 +/- 1.7 nl/min, respectively). In salt-deplete rats, endogenous tubular fluid and Ringer solution produced feedback responses of similar magnitude (EPF40-0 = 14.2 +/- 1.8 and 13.0 +/- 2.0 nl/min, respectively) but exogenous tubular fluid from salt-loaded rats elicited only a small feedback response (EPF40-0 = 1.5 +/- 1.6 nl/min), indistinguishable from that seen in salt-loaded rats with endogenous tubular fluid. It is concluded that an inhibitory factor in the tubular fluid of chronically salt-loaded rats causes a reduction in tubuloglomerular feedback response.