Na+ transport across the lamprey erythrocyte membrane was examined using 22Na as a tracer. Both Na+ influx and Na+ effux exhibit a wide variability among different lampreys due to amiloride-sensitive components. Addition of 1 mmol/l amiloride to incubation media resulted in a decrease of the Na+ influx from 8.4 +/- 0.9 to 5.5 +/- 0.3 mmol/l cells/h (n = 18, P < 0.001), and of the rate coefficient of the Na+ efflux from 0.50 +/- 0.08 to 0.18 +/- 0.02 h-1 (n = 20, P < 0.001). Cell shrinkage induced by addition of 100 mmol/l sucrose to an isotonic medium was associated with a significant increase in both the Na+ influx and the Na+ efflux which was entirely blocked by amiloride. The amiloride-sensitive components of the Na+ fluxes were abolished by cell swelling in hypotonic media (210 mosm/kg water). In addition to activation of Na+ influx by isoproterenol reported earlier (Gusev et al. 1992b), the present study demonstrated that isoproterenol also stimulated Na+ efflux from the lamprey erythrocytes. Exposure of the red cells to an Na(+)-free medium (replacing with NMDG+) resulted in a significant enhancement in Na+ efflux (approximately 3-fold) which was completely inhibited by amiloride. Incubation of the red cells in an unbuffered saline was accompanied by a gradual raising of external pHe measured with a pH-sensitive electrode, and the addition of 100 mmol/l sucrose to isotonic medium had no effect on pHe. The amiloride-sensitive component of the Na+ influx in the red cells incubated in the isotonic medium was unchanged when pHe was lowered from 7.4 to 6.5 but it disappeared after raising pHe to 8.0. The results of this study show that the lamprey erythrocyte membrane exhibits an amiloride-sensitive transport in either direction which is activated by cell shrinkage and isoproterenol, and is attenuated by cell swelling. No evidence was obtained for the contribution of well known Na(+)-H+ and Na(+)-Na+ exchangers to this amiloride-sensitive pathway under the conditions of our experiments.