The effect of lanthanum (La
3+) on Na
+ and Cl
– fluxes and on gill potentials was investigated in the goldfish,
Carassius auratus. Initially La
3+ caused large increases in both influx and efflux but after 15 min these decreased with influx returning to near normal levels while effluxes remained elevated, leading to a significant net loss of ions. Both La
3+ and Ca
2+ influenced gill potentials in an identical way. Possible physiological effects of La
3+ on gill ion exchange mechanisms are discussed.