The present work concerns mechanisms of ionic conductivity of photosynthetic membranes. It is shown that reconstitution of vesicles of photosynthetic membranes (chromatophores) of purple bacteria Rhodospirillum rubrum into a planar bilayer lipid membrane leads to fluctuations of current showing the existence of a channel with a predominant conductance of approximately 230 pS in the presence of 100 mM KCl. Measurements under the conditions of KCl gradient prove that this channel is cation selective (PK/PCl = 7.2). Voltage inactivation of the channel is demonstrated which is prevented by treatment with trypsin.