Significance: This paper describes the finding that mitochondria synthesize and release melatonin and have their selective G protein-coupled receptor (GPCR) in the outer membrane. We further demonstrate that mitochondrial melatonin type 1 receptors respond to melatonin by activating heterotrimeric G proteins located in the intermembrane space and inhibit stress-mediated cytochrome c release. This remarkable insight changes our classical understanding of biological GPCR function by showing that a cellular organelle both synthesizes and has a signaling receptor for a specific ligand. Implicit with our original work is the existence of an automitocrine signaling pathway by which melatonin prevents neurodegeneration associated with mitochondrial cytochromecrelease and downstream caspase activation.