In response to visual stimulation, a subset of neurons in the striate and prestriate cortex displays synchronous rhythmic firing in the gamma frequency band (20 to 70 hertz). This finding has raised two fundamental questions : What is the functional significance of synchronous gamma-band activity and how is it generated? This report addresses the second of these two questions. By means of intracellular recording and staining of single cells in the cat striate cortex in vivo, a biophysically distinct class of pyramidal neuron termed "chattering cells" is described. These neurons are located in the superficial layers of the cortex, intrinsically generate 20- to 70-hertz repetitive burst firing in response to suprathreshold depolarizing current injection, and exhibit pronounced oscillations in membrane potential during visual stimulation that are largely absent during periods of spontaneous activity. These properties suggest that chattering cells may make a substantial intracortical contribution to the generation of synchronous cortical oscillations and thus participate in the recruitment of large populations of cells into synchronously firing assemblies.