Several bivalve mollusc species possess neu- rohemoglobin in their nervous systems whereas most species do not. The cerebro-visceral connectives of Tel lina alternata and Spisula solidissima with neurohemo- globin and Tagelus plebeius and Geukensia demissa without neurohemoglobin exhibit similar electrical char acteristics dictated mostly by axon size (0.3-0.4 ^m mean axon diameter, Kraus et al, 1988). Action poten tial conduction is sensitive to a depletion of both ambi ent and neurohemoglobin-bound oxygen. Connectives without neurohemoglobin and connectives with carbon monoxide neurohemoglobin ceased to conduct action potentials within 5-10 minutes after exposure to anoxic conditions. Connectives with neurohemoglobin con ducted action potentials throughout the time course of neurohemoglobin deoxygenation, lasting 20-30 min utes. Connectives without neurohemoglobin exhibited an approximate five-fold elevation in oxygen consumption rate during action potential conduction, as predicted by axon diameter. However, connectives with neurohemo globin consumed only l/i this amount of oxygen during electrical activity. The mechanism for this increased efficiency in action potential conduction is unknown, but the rate of oxygen consumption nearly matches the rate of neurohemoglobin oxygen unloading in situ. An operational aerobic nervous system might enable ani mals to maintain neuromuscular activity during hypoxic or anoxic conditions.