Hippocampal neurons cultivated for the days indicated (DIV) were preloaded with [³H]GABA as given in Materials and Methods. Stimulation was performed by adding 0.5 mm 4-AP, by elevating the K⁺ concentration to 25 mm, or by applying 0.5 mm 4-AP in the presence of 25 mm K⁺. Secretion was stopped after 5 min at 37°C. Radioactivity was analyzed in the supernatant and the lysate of cells. Release is calculated as percent of the amount of radioactivity present at the beginning of stimulation. Values are given as percent of control being 2.7 ± 1 and 3.4 ± 1.2% in 7 or 21 DIV cultures, respectively, and represent the mean ± SD of three individual cultures.