Temperature-jump studies with fluorescence detection were made of the interactions of cAMP and its analogs with the cAMP receptor protein of Escherichia coli (CRP) which had been covalently labeled with N-(iodoacetylaminoethyl)-1-naphthylamine-5-sulfonate (1,5-I-AENS). A single relaxation process was observed for a solution containing the labeled CRP (AENS-CRP) in the absence of cyclic nucleotides. The relaxation time of this process is independent of the concentration of AENS-CRP, suggesting an interconversion between two isomeric forms of AENS-CRP. Two relaxation processes were observed for solutions containing AENS-CRP and cAMP. While the faster process reflects the bimolecular binding reaction, the slower process, which is apparently the same as that observed in the absence of cAMP, is associated with the conformational transition of AENS-CRP. The reciprocal relaxation time of the slower process decreases with increasing cAMP concentration, and can be quantitatively analyzed in terms of mechanism in which two different conformational forms of CRP are in equilibrium and the preferential binding of cAMP to one form leads to a shift of the equilibrium to this form. We propose that the form which favors cAMP is the biologically active form required for promoting gene transcription. This hypothesis is consistent with the finding that tubercidin 3′,5 ′-monophosphate, which is as effective as cAMP in stimulating transcription of the gal operon, also shifts the conformational equilibrium toward the active isomer. On the other hand, cGMP, a competitive inhibitor of cAMP, prevents cAMP from affecting the conformational transition. Thus the conformational transitions of CRP may play an important role in regulation of gene transcription.