Photosensitized inactivation of ionic channels formed by gramicidin in the planar bilayer lipid membrane (BLM) has been studied upon exposure of the BLM to single flashes of visible light in the presence of tetrasulphonated aluminium phthalocyanine. The gramicidin photoinactivation is inhibited by the addition of unsaturated phospholipids to the membrane-forming solution as well as by the addition of azide to the bathing solution, consistent with involvement of singlet oxygen. The characteristic time of the photoinactivation (tau) does not change markedly under these conditions. Moreover, tau remains nearly constant upon alteration of the flash energy and the photosensitizer concentration. The value of tau appears to be sensitive to the gramicidin concentration and to the factors affecting the open time of the gramicidin channels, namely the temperature and the solvent used in the membrane-forming solution. The photoinactivation is not observed with covalent gramicidin dimers. The equations derived from the model of Bamberg and Laeuger (J. Membrane Biol. (1973) 11, 177-194), describing the relaxation of the gramicidin-induced conductance after a sudden distortion of the dimer-monomer equilibrium, are shown to explain consistently the time course of the photoinactivation provided that the damage of the gramicidin molecules leads to deviation from the equilibrium.