We have analyzed the postnatal evolution of the spontaneous electrical activity in pyramidal neurons from rabbit hippocampal slices. The firing mode of CA1 neurons changes from bursting to regular spiking along the first postnatal month. Interspike intervals (ISIs) were used to account for the dynamical structure of the firing behavior. Histograms and joint interval scattergrams show that the firing mode from (P0-P7) cells has a different distribution from that obtained in (P15-P25) neurons. We have used a mathematical measure called the product of inertia to quantify this difference. Our findings demonstrate that the spontaneous activity changes along the maturational process.