The biologically active photoproduct of rhodopsin, metarhodopsin II (M II), exists in a pH-sensitive equilibrium with its precursor, metarhodopsin I (M I). Increasing acidity favors M II, with the midpoint of the pH titration curve at pH 6.4. To test the long-standing proposal that histidine protonation regulates this conformational transition, we characterized mutant rhodopsins in which each of the 6 histidines was replaced by phenylalanine or cysteine. Only mutants substituted at the 3 conserved histidines showed abnormal M I-M II equilibria. Those in which His-211 was replaced by phenylalanine or cysteine formed little or no M II at either extreme of pH, whereas mutants substituted at His-65 or at His-152 showed enhanced sensitivity to protons. The simplest interpretation of these results is that His-211 is the site where protonation strongly stabilizes the M II conformation and that His-65 and His-152 are sites where protonation modestly destabilizes the M II conformation.