A theoretical AM1 semiempirical study of the major active components of marijuana and their stereoisomers is presented. It was found that the calculated partition coefficients, dipole moments, heats of formation, volume, surface area, ovality, and conformation of the pyran ring cannot explain the activity differences between the stereoisomers. The major factor is the orientation of the carbocyclic ring and its C substituent with respect 1 to the phenyl group hydroxyl oxygen. Our study has revealed and supports the involvement of previously described steric features of cannabinoids in determining their biological potency. Based on these conclusions, we predicted the relative activities for the Ž .