The development of spontaneous bioelectric activity (SBA) in organotypic tissue cultures (OTCs) from rat occipital cortex was studied by means of extracellular recording techniques in OTCs grown normally for 6-51 days in vitro (DIV), and in OTCs in which SBA had been silenced from DIV 4 on for 2 to 3 weeks by elevating the Mg(2+) levels in the growth medium. The proportions of spontaneously active neurones increased from about 25% at 6-14 DIV to more than 80% beyond the third week in vitro. Mature neurones discharged at shorter intervals and more vigorously than immature neurones; the developmental increase in firing rate was not significant, however. In OTCs 6-14 DIV the majority of spontaneously active neurones fired sluggishly in a regular manner. The remaining neurones fired action potentials in the form of discrete bursts resembling interictal activity in vivo. The proportions of these neurones increased from about 40% at 6-14 DIV to more than 80% beyond the third week in vitro. During development in vitro the mean burst duration increased from 3.5 s to about 8 s whereas the mean burst rate (between 0.7-1 bursts/min) remained constant. Activity-deprived neurons had low firing rates and fired action potentials in the form of discrete bursts with a mean burst rate of 0.4/min. The proportions of spontaneously active neurons, the variability of neuronal firing and the viability of the explants either were not altered by the activity blockade or had recovered to control values after 5-6 days in normal growth medium. We conclude that in OTCs of rat neocortex the absence of SBA during development in vitro delays the maturation of excitatory mechanisms responsible for the developmental increase in firing intensity. The development of burst firing modes is less affected by activity blockade.