1. Two distinct forms of spontaneous synchronous oscillations were investigated with field potential recordings in slices of rat somatosensory cortex in vitro. 2. The first type of synchronous oscillation was activated by low extracellular [Mg2+] and had dominant frequencies of 8-12 Hz. It was abolished reversibly by the N-methyl-D-aspartate (NMDA) receptor antagonists D-2-amino-5-phosphonovaleric acid and was relatively unaffected by the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). The duration of oscillatory events was increased by blocking gamma-aminobuturic acid-A receptors with bicuculline or by activating metabotropic glutamate receptors with trans-1-aminocyclopentane-1,3-dicarboxylic acid. 3. A second form of synchronous oscillation was activated by acute application of kainic acid (10 microM), had dominant frequencies of 1-5 Hz, and was abolished reversibly by DNQX. Low concentrations of domoic acid mimicked the effects of kainate, but alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid or quisqualic acid did not, suggesting a role for the GluR5-7 and KA1-2 glutamate receptor subunits. 4. Surgical isolation of cortical layers showed that spontaneous NMDA receptor-dependent oscillations originated within layer 5 exclusively, but kainate receptor-dependent oscillations were uniquely generated by neurons in layers 2/3. 5. Our results suggest that neocortical neurons in layers 2/3 and layer 5 can independently generate two distinct forms of rhythmic population activity, each dependent upon activation of a different subtype of glutamate receptor.