Two critical amino acids in the visual pigment rhodopsin are Lys-296, the site of attachment of retinal to the protein through a protonated Schiff base linkage, and Clu-113, the Schiff base counterion. Mutation of Lys-296 or Clu-113 results in constitutive activation of opsin, as assayed by its ability to activate transducin in the absence of added chromophore. We conclude that opsin is constrained to an inactive conformation by a salt bridge between Lys-296 and Glu-113. Recently, one of the mutants, K296E, was found in a family with retinitis pigmentosa, suggesting that degeneration of the photoreceptor cells in individuals with this mutation may result from persistent stimulation of the phototransduction pathway.