Following the seminal work on the squid giant axon by Hodgkin & Huxley (HH; (1952) J. Physiol. 117, 500), action potentials in non-myelinated axons are generally assumed to be metabolically expensive (Attwell & Laughlin (2001) J. Cereb. Blood Flow Metab. 21, 1133) due to the inefficient overlap of underlying in- and outward currents. Here we show by recordings at physiological temperatures (36–37 °C) from presynaptic expansions of the non-myelinated mossy fiber in rat hippocampus, that during action potentials (250 ms half-duration) sodium and potassium currents are largely separated by rapid sodium current cessation and precisely matched delay of potassium current activation (100 ms). We find experimentally and in ion channel model-independent simulations reconstituting recorded action potentials that the efficiency of invested energy (minimum theoretical charge transfer to reach the action potential peak, C_m•DU, divided by the total sodium charge transfer per action potential) is 0.63 compared to 0.22–0.34 as predicted by conventional HH descriptions. Action potential reconstitutions in simulations using increased sodium conductance (gNa) decay times and decreased potassium conductance onset delays (to gNa onset) decrease the efficiency down to 0.24. The total sodium charge transfer along the mossy fiber associated with an action potential amounts to around 14 pC, corresponding to 0.7 pmol ATP/cm² (required by Na-K-ATPases to redistribute ions). This low metabolic cost per action potential shifts the emphasis regarding energy usage on downstream synaptic signalling, as indicated by a 6-fold larger total postsynaptic charge transfer at the mossy fiber output synapses onto principal neurons and interneurons, which was calculated on the basis of unitary AMPA receptor-mediated excitatory postsynaptic currents (Alle & Geiger (2006) Science 311, 1290; Geiger et al. (1997) Neuron 18, 1009) and morphological data (Acsady et al. (1998) J. Neurosci. 18, 3386). Thus, the mechanisms underlying physiological action potentials in mossy fibers are highly optimized to limit energy consumption (Laughlin & Sejnowski (2003) Science 301, 1870).