The formation of an intramolecular contact between distant residues is,perhaps,the simplest among the elementary processes characterizing the dynamics of unstructured polypeptide chains.Quenching of the triplet state oftryptopharn by close contact with cysteine has provided a new tool for measuring the rate of forming a specific contact in disordered polypeptides and in the unfolded state of proteins.From the viscosity dependence of the observed triplet lifetime after nanosecond UV laser excitation,both equilibrium (reaction-limited) and dynamic (diffusion-limited) contact formation rates are extracted in various polypeptides containing a single tryptophan and a single cysteine.