Autapses are transmitter release sites made by the axon of a neuron on its own dendrites. We determined the numbers and precise subcellular position of autapses on different spiny and smooth dendritic cell types using intracellular biocytin filling in slices of adult neocortex. Potential self-innervation was light microscopically assessed on 10 pyramidal cells, 7 spiny stellate cells, and 41 smooth dendritic neurons from cortical layers II-V. Putative autapses occurred on each smooth dendritic neuron and on seven pyramids, but not on spiny stellate cells. However, electron microscopic examination of all light microscopically predicted sites on pyramids (n = 28) showed only one case of self-innervation with two autapses on dendritic spines. Interneurons were classified by postsynaptic target distribution () and all putative autapses of seven basket, three dendrite-targeting, and three double bouquet cells were scrutinized. All basket and dendrite-targeting cells established self-innervation, the number of autapses being 12 +/- 7 and 22 +/- 12 (mean +/- SD), respectively; only one of the double bouquet cells formed autapses (n = 3). Basket cell autapses (n = 74) were closer to the soma (12.2 +/- 22.3 microm) than autapses established by dendrite-targeting cells (51.8 +/- 49.9 microm; n = 66). The degree of self-innervation is cell type-specific. Unlike on spiny cells, autapses are abundant on GABAergic basket and dendrite-targeting interneurons, with subcellular location similar to that of synapses formed by the parent cell on other neurons. The extensive self-innervation may modulate integrative properties and/or the firing rhythm of the neuron in a manner temporally correlated with its own activity.