1. The inhibition of F1-ATPase by its natural peptide inhibitor is mixed non-competitive with two pH optimum values (5.5 and 8.2). 2. A two-step model for the interaction is suggested in which two enzyme conformations would exhibit different affinities for the peptide. 3. At low pH, interaction would be favoured. At high pH, a conformation (not susceptible to inhibition) changes into another (susceptible to inhibition) through the hydrolytic reaction stimulation, due to high pH.