Hippocampal pyramidal neurons are exposed to extracellular electric fields in vivo, and we are interested in the impact of electric fields on activities of the neurons. Here, we investigated whether the weak fields affect spike initiation, spike propagation, and [Ca²⁺]_i elevation in the pyramidal neurons. We performed whole-cell recordings from the soma of CA1 pyramidal neurons in acute rat hippocampal slices, and loaded the neurons with fluorescent Ca indicator dye Oregon Green 488 BAPTA-1. The fluorescent change was recorded from the dendrites and the soma with a CCD camera while the membrane potential simultaneously recorded from the soma. A pair of Ag-AgCl electrode were positioned on either side of the slice to load electric fields of the somato-dendritic axis of the cell. To examine the impact of electric fields on spike initiation and propagation, we gave a short current pulse to the soma to trigger action potentials (AP) and applied electric fields. When we applied electric fields with intensity 40 mV/mm which depolarized the soma side, the APs were initiated on the soma and [Ca²⁺]_i elevation were observed with propagation to the dendrites. When weak electric fields with intensity 20mV/mm was applied in a direction that hyperpolarize the soma, a short current injection did not initiate AP and [Ca²⁺]_i elevation was not observed. When we applied electric fields in a direction that depolarize the distal part of the apical dendrites at intensity as high as 80mV/mm in normal ACSF, [Ca²⁺]_i elevation was not observed in the apical dendrites. Only under higher extracellular K⁺ condition (4.5-5.5mM), electric fields with intensity 80mV/mm induced [Ca²⁺]_i elevation in the distal apical dendrites. These results indicate that electric fields as weak as 20 mV/mm can affect spike initiation, although electric fields with intensity as intense as 80 mV/mm does not evoke [Ca²⁺]_i elevation by itself in the apical dendrites.