Previous studies have demonstrated that stop-flow pressure (SFP) feedback responses can occur during orthograde perfusion with solutions having low amounts of sodium or chloride. However, retrograde perfusion studies have suggested a specific role for chloride concentration in mediating feedback responses. These studies were conducted to compare SFP feedback responses during orthograde and retrograde perfusion with an artificial tubular fluid solution (ATF) (Cl - = 135 meq/liter) and a Na + isethionate solution (Cl - = 6 meq/liter). With ATF, increases in perfusion rate from 10 to 35 nl/min led to decreases in SFP of 11 ± 1.4 mmHg, increases in distal tubular fluid Cl - of 46 ± 4.9 meq/liter, and osmolality of 58 ± 10 mosmol/kg. There were significant inverse relationships between SFP and changes in Cl - and osmolality. With Na + isethionate, SFP decreased by 8.4 ± 1.0 mmHg, osmolality increased by 43 ± 8 mosmol/kg, and Cl - did not change. There was a significant relationship between SFP and osmolality, but not with Cl - . During retrograde perfusion at 15 nl/min, SFP decreased by 12 ± 1.2 mmHg with ATF and by 12 ± 1.2 mmHg with Na + isethionate. These results demonstrate that feedback-mediated decreases in SFP can occur in the absence of concomitant increases in distal Cl - and suggest that the receptor system does not have a unique and specific requirement for chloride.