The ontogeny of presynaptic elements of GABAergic neurons has been studied in the cerebrum of the chick embryo both in vivo and in vitro. The specific activity of glutamate decarboxylase (GAD) in tissue extracts followed a rising curve and approached a plateau value after 28 days in vivo. One-half of the adult levels of GAD were achieved by day 20. The specific activity of Na⁻-γ-aminobutyric acid (GABA) cotransport in membrane vesicles followed a similar pattern and reached maximal levels by 28 days in vivo. One-half of the adult levels of GABA uptake were observed at day 17. The development of these markers was also studied in cultured neurons prepared from the cerebrum of 8-day-old chick embryos. The GAD activity in neuronal extracts increased linearly with time in culture up to 14 days. At this point the specific activity had reached 20% of that observed for the adult cerebrum. The specific activity of GABA uptake by intact neurons followed a pattern similar to that for GAD from days 2 to 9 in culture. Both activities increased 4–5-fold during this period, but the level of GABA transport declined thereafter. In order to compare GABA uptake values for cultured cells with those for embryonic and adult brain, membrane vesicles were prepared from cultures. At the maximal level (9–10 days in culture) the vesicular GABA uptake represented 33% of that in the 18-day embryo and 20% of adult levels. Thus the presynaptic GABAergic components developed according to similar schedules both in vivo and in vitro. This pattern was also in correspondence with the ontogeny of postsynaptic elements of the GABA_A receptor complex in the same preparations, as described in the companion paper.