By introducing a photolabile group in the Watson-Crick base-pairing site of a guanosine residue, a bistable 20-base RNA sequence was forced into a less stable conformation. A single laser pulse released the native sequence, and the subsequent refolding equilibration was monitored with time-resolved NMR spectroscopy. This permitted a quant. description of the refolding process. [on SciFinder (R)]