Coelenterazine emits light by chemi-and bioluminescence reactions, decomposing into coelenteramide and CO(2). To ascertain the light emitters involved, the fluorescence of coelenteramide and five analogues were studies in four kinds of solvent. The results showed that coelenteramides can form five kinds of light emitters, ie unionized (lambda(max) 386-423 nm), phenolate anion (lambda(max) 480-490 nm), phenolate anion temporarily formed from the ion-pair state (lambda(max) 465-479 nm), amide anion (lambda(max) 435-458 nm) and pyrazine-N(4) anion (lambda(max) 530-565 nm). The chemiluminescence light emitter of coelenterazine in the presence of alkali (lambda(max) 530-550 nm) was found to be the pyrazine-N(4) anion and not the dianion (ie phenolate anion/amide anion), as previously believed. In chemiluminescence, the normal light emitter is the amide anion, and the pyrazine-N(4) anion emission may occur in the presence of alkali, but light emission from any other emitters has not been observed. In the bioluminescence reaction, the normal light emitter is the amide anion, but no other light emitter was observed except the unionized form found in the Ca-triggered luminescence of semisynthetic aequorins prepared with an e-type coelenterazine instead of coelenterazine.