Many of the different morphological types of interneurons in mammalian neocortex are presumed to be inhibitory, but to date, conclusive functional data have been lacking. Using paired intracellular recordings in slices of adult rat somatosensory cortex, we present a sparsely spiny, burst firing interneuron that elicits in a simultaneously recorded pyramid a fast inhibitory postsynaptic potential, reversing at -78 mV. Neither inhibitory postsynaptic potential time course, nor paired pulse depression (inter-spike interval 15-120 ms), was affected by addition of the GABAB antagonist/partial agonist 2-OH-Saclofen (250 microM), but increasing extracellular [Ca2+] enhanced inhibitory postsynaptic potential amplitude at low firing rates and increased paired pulse depression at higher rates. Light microscopic examination of the biocytin-filled neurons revealed the presynaptic cell to be a sparsely spiny interneuron and the postsynaptic to be a small pyramidal neuron, both in layer II. Ultrastructural examination of 16 terminals of the presynaptic interneuron revealed that they formed symmetric contacts with unlabelled neurons, four with neuronal somata, 10 with dendritic shafts and two with spine shafts. This, therefore, is the first report of the properties of a single axon inhibitory postsynaptic potential in neocortex resulting from action potentials in an electro-physiologically and morphologically identified interneuron. We propose that at least some of the sparsely spiny, burst firing interneurons inhibit pyramidal neurons via GABAA receptors.