The effects of the psychoactive cannabinoid delta 9-tetrahydrocannabinol (THC) and the nonpsychoactive cannabinoid cannabidiol (CBD) were investigated comparatively on electrically caused transcallosal cortical evoked responses, electrically induced limbic after discharges, photically evoked cortical afterdischarges, spontaneous cortical focal epileptic potentials, and spinal monosynaptic reflexes. In each system, THC produced central excitation; for example, the drug's responses ranged from enhancement of synaptic transmission to precipitation of frank convulsions. In addition to central nervous system stimulation, THC usually elicited depression; the qualitative character of the effect of the drug was dependent upon the dosage and the test system. In contrast to THC, cannabidiol generated no CNS excitation: it was either depressant or inert in these test systems. The results clearly demonstrate the complexity of the CNS properties of THC and the selectivity of the depressant properties of cannabidiol; moreover, the data illustrate the wide range of neuropharmacologic responses that potentially any cannabinoid can effect.