We describe here the results obtained to date on a project aimed at characterizing the changes occurring in the protein product (p21) of the H-ras proto-oncogene during and as a result of hydrolysis of GTP at its active site. The approach used involves crystallization of p21 with a photosensitive precursor of GTP (caged GTP) at the active site followed by generation of GTP by photolysis and collection of X-ray diffraction data using the Laue method at a synchrotron source. The structure of p21 complexed with a single diastereomer of caged GTP is presented here. In contrast to crystals obtained with mixed diastereomers, the nucleotide appears to bind in a manner which is very similar to that of other guanine nucleotides (GDP, GTP, GppNHp). The current state of time resolved structural experiments using these crystals is presented.