Structure-based drug design relies on static protein structures despite significant evidence for the need to include protein dynamics as a serious consideration. In practice, dynamic motions are neglected because they are not understood well enough to model, a situation resulting from a lack of explicit experimental examples of dynamic receptor ligand complexes. Here, we report high-resolution details of pronounced similar to 1 ms time scale motions of a receptor small molecule complex using a combination of NMR and X-ray crystallography Large conformational dynamics in Escherichia coli dihydrofolate reductase are driven by internal switching motions of the drug-like, nanomolar-affinity inhibitor. Carr-Purcell-Meiboom-Gill relaxation dispersion experiments and NOEs revealed the crystal structure to contain critical elements of the high energy protein-ligand conformation. The availability of accurate, structurally resolved dynamics in a protein ligand complex should serve as a valuable benchmark for modeling dynamics in other receptor ligand complexes and prediction of binding affinities.