A combination of NMR spectroscopy and molecular modeling has been employed to characterize the conformation and dynamics of the macrolide ring in verrucarin A and roridin A, two closely related toxins in the trichothecene mycotoxin family. Longitudinal carbon-13 relaxation times demonstrate the relative flexibility of the macrolide ring. The calculations, NOEs, and scalar vicinal coupling constants show that verrucarin A in CDCl 3 and CD 2Cl 2 predominantly adopts a single, well-defined conformation that matches the crystal structure. In contrast, roridin A is present as a mixture of two conformers.