The bioluminescence of the ctenophore Mnemiopsis takes place when the photoprotein mnemiopsin in the photocytes reacts with Ca2+. The luminescence is inhibited in sunlight and this photoinhibition is reversible by keeping the live specimens in the dark. The extracts of mnemiopsin are similarly photoinhibited, but the photoinhibition cannot be reversed in the dark. We have found that photo-inhibited mnemiopsin can be re-activated in the dark by incubation with coelenterazine and O2 only in solutions having a pH very close to 9.0. The re-activation in vivo probably takes place in the same manner, using the coelenterazine that is supplied from its abundant storage form. Various lines of experimental evidence suggest that the photoinactivation of mnemiopsin results in the dissociation of coelenterazine and oxygen from the molecule of photoprotein; the dissociated form of the former molecule is an inactive form of coelenterazine, not free coelenterazine.