The essential oil of "Nepeta cataria" L. (catnip), an herbaceous plant known popularly as a stimulant for the domestic cat, is enriched in two diastereomeric iridoid lactones (nepetalactones) that vary in ratio. Although the diastereomers are chromatographically separable and exhibit different 1D NMR spectra, it is not possible to differentiate between them chromatographically or spectroscopically without previous knowledge or more complex NMR experiments; however, the trans-fused lactone readily epimerizes to the thermodynamically more stable cis-fused lactone upon treatment with base. Students in the introductory organic laboratory establish the presence of the two major nepetalactones in catnip essential oil by TLC and GC-MS analysis, assigning TLC spots and GC peaks to specific lactones by chromatographic comparison of the essential oil with the product of epimerization (i.e., cis-fused lactone) and determining the diastereomeric ratio of nepetalactones by GC peak integration. (Contains 4 figures, 1 scheme, and 1 table.)