Dentate gyrus granule cells transmit action potentials (APs) along their unmyelinated mossy fibre axons to the CA3 region. Although the initiation and propagation of APs are fundamental steps during neural computation, little is known about the site of AP initiation and the speed of propagation in mossy fibre axons. To address these questions, we performed simultaneous somatic and axonal whole-cell recordings from granule cells in acute hippocampal slices of adult mice at ~23°C. Injection of short current pulses or synaptic stimulation evoked axonal and somatic APs with similar amplitudes. By contrast, the time course was significantly different, as axonal APs had a higher maximal rate of rise (464 ± 30 V/s in the axon vs. 297 ± 12 V/s in the soma, mean ± SEM). Furthermore, analysis of latencies between the axonal and somatic signals showed that APs were initiated in the proximal axon at ~20-30 µm distance from the soma, and propagated orthodromically with a velocity of 0.24 m/s. Qualitatively similar results were obtained at a recording temperature of ~34°C. Modelling of AP propagation in detailed cable models of granule cells suggested that a ~4 times higher Na+ channel density (~1000 pS µm-2) in the axon might account for both the higher rate of rise of axonal APs and the robust AP initiation in the proximal mossy fibre axon. This may be of critical importance to separate dendritic integration of thousands of synaptic inputs from the generation and transmission of a common AP output.