Extracellular electrical stimulation of the gray matter is often used to determine the function of a given cortical area or pathway. However, when it is used to elicit postsynaptic effects, the presynaptic neuronal elements activated by electrical stimulation have never been clearly identified: it could be the excitable dendrites, the cell body, the axon initial segment, or the axonal branches. To identify these elements, we performed two series of experiments on slices of rat visual cortex maintained in vitro. The first series of experiments, reported in this paper, was aimed at determining the chronaxie, a temporal parameter related to the membrane properties of the neuronal elements. In order to identify the presynaptic elements that were activated by extracellular electrical stimulation, chronaxies corresponding to postsynaptic responses were measured and compared with those corresponding to the activation of axons (antidromic activation) and those corresponding to the activation of cell bodies (intracellular current injection in intracellularly recorded neurons). The chronaxie for orthodromic activation was similar to that for axonal activation, but was 40 times smaller than the chronaxie for direct cell body activation. This suggests that, whenever a postsynaptic response is elicited after electrical stimulation of the cortical gray matter, axons (either axonal branches or axon initial segments), but not cell bodies, are the neuronal elements activated.