It　was　found　that　melatonin　and　its　derivatives,　such　as　N-acetyl-　5-methoxytryptamine　(MT),　N-acetyl-5-hydroxytryptamine　(NAS),　5-Methoxytrypt-　amine　(5-MT),　5-Methoxyindolyl　acetic　acid　(5-MIAA)　and　N-acetyl-5-methoxy-　6-hydroxytryptamine　(6-HMT)　would　give　chemiluminescence　in　the　acidic　potassium　permanganate　solution,　and　formaldehyde　would　enhance　this　chemiluminescent　reaction　greatly.　The　optimum　conditions　for　this　chemiluminescent　reaction　were　studied　in　detail　by　a　flow　injection　system.　A　new　simple　rapid　method　has　been　developed　under　the　optimum　conditions　for　determination　of　melatonin.　This　method　has　the　advantages　of　high　sensitivity,　wide　range　of　linear　response　and　low　detection　limit.　On　the　basis　of　investigation　of　chemiluminescent,　fluorescent　and　UV　spectra　of　melatonin　in　acidic　solution　containing　potassium　permanganate　and　formaldehyde,　a　possible　mechanism　of　this　reaction　was　proposed.