Slices (n = 45) from the somatosensory cortex of mouse (P8-13) generated spontaneous bursts of activity (0.10 +/- 0.05 Hz) that were recorded extracellularly. Multiunit action potential (AP) activity was integrated and used as an index of population activity. In this experimental model, seizure-like activity (SLA) was evoked with bicuculline (5-10 microM) or N-methyl-d-aspartate (NMDA, 5 microM). SLA was an episode with repetitive bursting at a frequency of 0.50 +/- 0.06 Hz. To evaluate whether SLA was associated with a change in synchrony, we obtained simultaneous intracellular and extracellular recordings (n = 40) and quantified the relationship between individual cells and the surrounding population of neurons. During the SLA there was an increase in population activity and bursting activity was observed in neurons and areas that were previously silent. We defined synchrony as cellular activity that is consistently locked with the population bursts. Signal-averaging techniques were used to determine this component. To quantitatively assess change in synchronous activity at SLA onset, we estimated the entropy of the single cell's spike trains and subdivided this measure into network burst-related information and noise-related entropy. The burst-related information was not significantly altered at the onset of NMDA-evoked SLA and slightly increased when evoked with bicuculline. The signal-to-noise ratio determined from the entropy estimates showed a significant decrease (instead of an expected increase) during SLA. We conclude that the increased population activity during the SLA is attributed to recruitment of neurons rather than to increased synchrony of each of the individual elements.