Evoked inhibitory postsynaptic potentials (IPSPs) were studied in CA3 hippocampal neurons from brain slice preparations of rats ranging from 5 to 18 days of age (P5–18) using intracellular recording techniques. With KMeSO4-filled electrodes the evoked inhibitory response consisted of fast and slow IPSPs mediated by GABAA and GABAB receptors respectively. In recordings obtained with electrodes filled with 2-(triethylamino)-N-(2,6-dimethylphenyl) acetamide and KMeSO4, electrical stimulation evoked monophasic IPSPs in mature slices (P10–18) and biphasic IPSPs with an early and a late phase in neonatal slices (P4–7). In neonates both the early and late phases of the IPSP were mediated by GABAA receptors. Pharmacological investigation revealed that the early phase arose from both direct and feedforward activation of GABAergic interneurons involving non-NMDA receptors, while the late phase resulted from polysynaptic activation of GABAergic interneurons mediated by NMDA receptors.