The present research was designed to determine if a single LSD experience produces a detectable change in the electrical potential of the brain, and if so, how long that change lasts, and to examine the changes in the electrical potential of the brain associated with chemogenic and hypnogenic hallucinations. Four studies were conducted measuring the electrical potential between the front and back of the head in monkeys and humans. The frontal potential was shown to shift negatively with the use of LSD-type psychedelics but not with marijuana compounds. LSD caused the potential in humans to shift into the range commonly exhibited by hallucinating schizophrenics. LSD-25 in a monkey caused an extreme negative shift, while brom-LSD caused a small negative shift, and THC caused no shift. There were no consistent shifts associated with hypnosis or hypnotic phenomena such as hypnogenic hallucinations or hypnoanalgesia. The results indicated: that LSD, unlike marijuana compounds, caused a long-lasting but limited neurochemical change in the user; that these neurochemical changes were not due to hallucinations per se but may or may not have accompanied them; and that chemogenic hallucinations and hypnotic hallucinations were not based on a common physiological mechanism