Our experiments suggest that LSD has an inhibitory effect on biosynthesis and/or the release of serotonin from nerve terminals, which would lead to a decrease in brain serotonin turnover and inhibitioin of serotonin-containing neurones. As for DMT, it is believed to be a potent inhibitor of the uptake of serotonin and the effect of raphe stimulation on the serotonin and 5-HIAA in the rat forebrain might be explained on this basis. If the serotonin released by stimulation were not taken up and metabolized by monoamine oxidase, this would account for the observed increase in the forebrain serotonin content. Fuerther, DMT may interfere with the release of serotonin.