Although several studies have compared hippocampal slices from young vs adult rats, a systematic in vivo characterization of the ontogeny of electrophysiologic responses in this structure has not been done. The current report describes the postnatal development of excitatory and inhibitory responses in the CA1 region of the hippocampus in rats 7-65 days of age. Under urethane anesthesia, a recording electrode was placed in one CA1 region to measure extracellular population spikes elicited by stimulation of the contralateral CA3 region. Age-related changes in the maximal population spike amplitude, the voltage required to elicit a half-maximal amplitude spike, the width at half-maximal spike amplitude, and latency to onset of the population spike ('conduction velocity') were monitored as parameters describing excitatory processes in the hippocampus. A paired-pulse paradigm was used to quantify the ontogeny of inhibitory processes. In younger animals, population spikes were broader, smaller in amplitude, and required higher stimulus intensities to be elicited. After postnatal (PN) day 14, excitability (voltage to elicit half-maximal population spike) and spike width were at fully mature levels. Maximal spike amplitudes were also smaller in rats younger than PN14, but not thereafter. The conduction velocity parameter steadily increased during development. In contrast, no evidence of inhibition was found prior to PN18, after which it steadily increased to reach adult levels by PN28. These results indicate that, in the rat hippocampus, excitatory processes are well established or fully mature within 2 weeks following birth, whereas the maturation of inhibitory processes to adult levels is not achieved until several weeks later.