Networks of cultured cortical neurones exhibit regular, synchronized, propagating bursts which are synaptically mediated, and which are hypothesized to play a part in activity-dependent formation of connections during development in vivo. The relationship between the strength of synaptic connections and the characteristics of synchronized propagating bursting, however, is unclear. Modification of synchronized activity in cortical cultures in response to electrical stimulation was examined using multisite electrode array recording. By measuring the response of the network to weak, localized, test stimulation (TS), we observed a potentiation of activity following a relatively stronger inducing stimulation (IS). This potentiation was evident as an increased probability of eliciting bursts by TS, an increased frequency of spontaneous bursts and number of spikes per burst, and increased speed of burst propagation, and it lasted for at least 20 min. Changing the parameters of IS revealed that high frequency tetanic stimulation is not necessary to induce potentiation, while it is essential for IS to produce a regeneratively propagating burst. The results provide a direct demonstration of modification of both the spatial and temporal characteristics of synchronized network activity, and suggest an important physiological role for propagating synchronized bursting, as a mechanism for inducing plastic modifications in the developing cortex.