The conformation for each of two cannabinoids, delta 9-tetrahydrocannabinol (THC), the principal active constituent of marihuana, and delta 9,11-THC, a biologically inactive structural isomer of delta 9- THC, is analyzed by interpreting 1H-1H and 13C-1H coupling constants and nuclear Overhauser effects determined using one-and two-dimensional 1H and 13C NMR techniques. The interpretation of both vicinal and long range coupling constants was necessary to deduce the conformations of these tricyclic molecules with certainty, with nuclear Overhauser effect enhancements used, when appropriate, to confirm the results. These findings provide insights into the structure-activity requirements for the cannabinoids.