The development of spontaneous bioelectric activity (SBA) was studied in dissociated occipital cortex cultures prepared from 19 day old rat fetuses. All cultures, recorded one per diem from 5 to 33 days in vitro (div), showed SBA. Computer analysis of 76 extracellularly recorded single unit spike trains was carried out after selection on the basis of stationarity criteria. Statistically significant developmental trends were found in (i) interspike interval dependencies and (ii) fluctuations in mean firing rate, on the order of a minute or longer. The highly dependent firing patterns, including stereotyped bursting, were present mostly in the 9-12 div group, whereas minute-to-minute fluctuations in the intensity of firing were considerably more pronounced in the oldest group (22-33 div) than in the younger cultures. In addition, firing categories defined on the basis of factor-analysis revealed that such fluctuations were almost exclusively to be found in neurons which fired in a pronounced 'burst', rather than a relatively continuous fashion. Only a few mature appearing synaptic structures were observed electron microscopically prior to 12 div, but increased steadily in number thereafter. No cultures prior to 14 div, but all cultures older than this, stained positively for the presence of glutamic acid decarboxylase. An extensive immunoreactive, putative GABAergic, network was present by three weeks in vitro.