Organoids derived from human stem cells present enormous potential to investigate aspects of human brain development. In this study, we hypothesised that brain organoids represent a suitable method for studying the shift in GABAergic transmission, from excitatory to inhibitory, during development. As several anaesthetics act via GABA receptors, we aimed to investigate how the anaesthetic propofol, known to be a GABA-A receptor agonist, may affect neuronal transmission in cerebral brain organoids at different developmental stages. Organoids cultivated from human embryonic stem cells (H1-ESC), with a putative cortical identity, were taken at day 60 and 90 and placed on a high-density multi-electrode array (HD-MEA). Electrical activity was recorded from 4096 electrodes in control conditions, in the presence of propofol (50µM) and separately of the GABA-A receptor antagonist picrotoxin (100µM). Spontaneous neuronal activity was observed in several channels, which was eliminated with addition of TTX (2µM), confirming the action potential origin of the recorded signals. Spike detection was performed using custom scripts and the 50 most active channels were selected for in-depth analysis. Application of PTX reduced activity in the 60 day organoids but increased activity at day 90, suggesting that GABAergic transmission is not yet fully mature at 60 days in this model. The effects of propofol were low at both ages, which is in agreement with other reports suggesting that propofol acts not only through GABAergic receptors, but at high concentrations may also act to inhibit cation channels. Supported by the German Research Foundation (CRC1080, C02) to T.M.