The fluorescence probe, 8-anilino-l-naphthalenesulfonic acid (ANS), was used to monitor the induced-fit conformational movement in rabbit muscle adenylate kinase. In 50 mM Tris-HCl buffer (pH 8.1), the time course of ANS binding to rabbit muscle adenylate kinase is a biphasic process. The fast phase completes within the dead-time of the stoppedflow equipment used (about 15 ms), while the slow phase ends in about 10 minutes. In the presence of 2.0 (iM peptidyl prolyl cisl rra/w-isomerase, the rate constant of the slow phase reaction is accelerated about 2.4-fold, suggesting that the domain movement during ANS binding to rabbit muscle adenylate kinase may involve proline isomerization. The activation energy of the slow phase was determined to be 74.6 kJ/mol, which is comparable to the activation energy of proline cis/frans-isomerization (about 80 kJ/mol).