Resting and stimulated fluxes of sodium and potassium across the giant axon of the marine annelid, Myxicola infundibulum, have been characterized using the technique of internal dialysis. In most respects the ion movements were found to be similar to those in squid axons. Sodium efflux and potassium influx were found to be active, cardiac glycoside-sensitive fluxes, with a variable coupling ratio. However, when [ATP]i was lowered to less than 20 microM by treatment with cyanide and continuous dialysis, or to less than 2 microM by dialysis with glucose following injection of hexokinase, Na efflux and K influx were unaltered. The maintained fluxes were not accounted for by an increased passive permeability of the axolemma, although 30-60% of the Na efflux appeared to be due to Na-Na exchange. An altered form of Na pump operation at low [ATP]i is a more likely explanation than an alternate energy source, or an ATP source proximate to the axolemma. The transient response of 22Na efflux to a change in [22Na]i was found to be much slower than in squid, tau = 360 sec. The efflux delay could only be accounted for by an extra-axonal diffusion barrier, which is probably the basement membrane surrounding the ventral nerve cord.