Endothelin 1 (ET–1) is a powerful constrictor of the afferent glomerular artery, implicated in the occurrence of both functional and acute renal failure. Besides being produced by the endothelium, ET–1 is also secreted by proximal tubular cells, suggesting that it may act as an endoluminal messenger. The present study is intended to verify whether ET–1 may play a role in the tubuloglomerular feedback system. The experiments were performed in rat superficial glomeruli. In 25 nephrons we measured by the total collection technique the single–nephron glomerular filtration rate (SNGFR; nl/min) and reabsorption rates before (control) and during microinjection (MIJ) of ET–1 10<sup>–7</sup> M into the first proximal convolution or Bowman’s space. The SNGFR rose from 27±3 to 61±11 nl/min (p<0.01), the percent proximal reabsorption rose from 43 to 74%, and the absolute reabsorption rose from 13±2 to 46±11 nl/min (p<0.01). In additional 23 nephrons the collections were performed at the earliest distal convolution accessible on the renal surface, while MIJ was performed in the last proximal convolution of the same nephrons. The SNGFR rose during MIJ from 22±3 to 40±6 nl/min (p<0.01), the percent reabsorption rose from 61 to 66% (p>0.77), and the absolute reabsorption rose from 12±2 to 26±4 nl/min, (p<0.003). Exposure of the macula densa to intraluminally injected ET–1 causes an abrupt increase in SNGFR of the experimental nephron, in the absence of changes in systemic and renal hemodynamics. During proximal MIJ, ET–1 may have reached the macula densa during the time preceding the beginning of collection and interruption of the delivery of fluid to the distal nephron. ET–1 directly stimulates fractional and absolute volume reabsorption along the proximal tubule. Proximal secretion and/or filtration of ET–1 could represent a physiological mechanism to activate the tubuloglomerular feedback, eliciting a response opposite to that triggered by systemic and intrarenal infusion.