Transient, high-frequency hippocampal oscillations called sharp wave–ripples may be involved in the formation of memories that are later consolidated in the neocortex. Oscillations of ripple frequency (∼200 Hz) have been reported in the rodent neocortex during sleep but are not well understood. Here, the authors made field-potential recordings from the neocortex of sleeping rats and found that short periods of ripple oscillations, produced by subpopulations of fast-spiking, parvalbumin-containing basket cells, occurred in the troughs of neocortical sleep 'spindles' (periods of low-frequency oscillations) and could play a part in memory processes.