We present a novel approach for non-invasive investigation of gamma-aminobutyric acid (GABA) actions in neuronal networks of cerebral cortex. It does not require using any pharmacological agents and preserves native intracellular ion composition as well as intact membrane potential of cells affected by GABA. Amice line expressing channelrhodopsin-2 (ChR2) in GABAergic interneurons was used to monitor developmental changes in GABA-mediated responses characteristic for rodents’ brain early in postnatal life. ChR2 is a light-gated cation-permeable membrane receptor which allows Na+ influx to depolarize and excite cells when illuminated by blue (~ 470 nm) light. Patch-clamp recordings were performed during laser stimulation of ChR2 expressing interneurons in hippocampal and neocortical slices obtained from P (postnatal day) 2-15 mice. Starting from P2 onwards, laser light excited interneurons to release GABA and to evoke postsynaptic GABAergic currents. Excitatory/inhibitory GABA actions on pyramidal cells were further assessed by monitoring EPSCs frequencyalteration during photoactivation of interneurons. Until P8, photoactivation of interneurons caused an increase in EPSCs frequency, whereas in older animals a decrease in EPSCs frequency was observed. Thus, the timing of the developmental switch obtained using this technique matches the developmental changes in GABA actions previously reported using traditional electrophysiological and imaging methods. This study demonstrates a utility of presented optogenetics based approach for investigation of GABA actions during development. It can also be employed for exploration of GABAergic function in other physiological or pathological conditions where an assessment of GABA actions is required.