It has previously been shown (Baroin, A,, F . Garcia-Romeu, T . Lamarre, and R . Motais . 1984a, b . Journal of Physiology . 350 :137, 356 :21 ; Mahe, Y ., F . Garcia-Romeu, and R . Motais . 1985 . European Journal of Pharmacology . 116 :199) that the addition of catecholamines to an isotonic suspension of nucleated red blood cells of the rainbow trout first stimulates a cAMPdependent, amiloride-sensitive Na'/H' exchange . This stimulation seems to be transient . It is followed by a more permanent activation of a coupled entry of Na' and Cl -, which is inhibited by amiloride but also by inhibitors of band 3 protein (DIDS, furosemide, niflumic acid) . The coupled entry of Na' and Clcould therefore result from the parallel and simultaneous exchange of Na .' for H (via the cAMP-dependent Na'/H' antiporter) and Clo-, for HCO3-; (via the anion exchange system located in band 3 protein) . However, in view of the following arguments, it had been proposed that NaCI uptake does not proceed by the double-exchanger system but via an NaCI cotransport : (a) Na' entry requires Cl -as anion (in NO-3 medium, the Na uptake is strongly inhibited, whereas NO-3 is an extremely effective substitute for Cl-in the anion exchange system) ; (b) Na uptake is not significantly affected by the presence of HCO-3 in the suspension medium despite the fact that in red cells, CI-/HCO-exchange occurs more readily than the exchanges of Cl -for basic equivalents in a theoretically C0 2-free medium (the so-called Cl -/OH-exchanges) . The purpose of the present paper was a reassessment of the two models by using monensin, an ionophore allowing Na'/H' exchange . From this study, it appears that NaCl entry results from the simultaneous functioning of the Na'/H' antiporter and the anion exchange system . The apparent Cl dependence is explained by the fact that, in these erythrocytes, NO-clearly inhibits the turnover rate of the Na'/H' antiporter . As Na'/H' exchange is the driving component in the salt uptake process, this inhibition explains the Cl requirement for Na entry . The lack of stimulation of cell swelling by bicarbonate is explained by the fact that the rate of anion exchange in a C0 2-free medium (Cl-/OH -exchange) is roughly equivalent to that of Na'/H' exchange and thus in practice is not limiting to the net influx of NaCl through the two exchangers . The acidification J . GEN . PHYSIOL .