The theta rhythm is the most remarkable hippocampal activity correlated with various physiological and behavioral phenomena. Although analyzed by numerous investigators, during the last five decades, the mechanisms that lead to its generation still remain reason of debate. In the present report it is shown that hippocampal neurons, recorded from juvenile and adult slices, are endowed with intrinsic properties that allow the generation of a steady oscillatory activity. The frequency of this rhythmic ongoing process is highly sensitive to the level of the membrane potential, reaching values in the theta range, up to 6 Hz, for more depolarized values of membrane potential. Membrane potential oscillations are unmasked by loading pyramidal neurons with intracellular cesium, are sodium-independent, and are generated by the sequential activation of calcium and potassium conductances. Finally, like theta rhythm, regularly occurring membrane potential oscillations can be detected since postnatal day 10 and their frequency increases with age, i.e., during the following 2 weeks.