Genetic methods for neuronal silencing have great promise for allowing selective inactivation of specific cell types within complex neural systems. Present methods, however, are limited in their reversibility by the slow time scale (days) of transcriptional regulation. We report the rapid and reversible inactivation of mammalian cortical neurons expressing the insect G-protein-coupled receptor AlstR (Drosophila allatostatin receptor) [corrected] after application of its peptide ligand allatostatin (AL). The onset and reversal of inactivation could be achieved rapidly, within minutes. Moreover, the effects of AL were selective for AlstR-transfected neurons. The AlstR/AL system is therefore a promising genetic method for selective and quickly reversible silencing of neuronal activity.