In practice, any chemical substance that is a normal constituent of nervous tissue and has a defined excitatory or inhibitory action on nerve or muscle cells is potentially classifiable as a neurotransmitter. In this chapter we have presented the data to date that illustrate that DMT is a normal constituent of mammalian brain and other tissues. Enzymes capable of synthesizing DMT from TA and NMT have also been described. These enzymes are apparently controlled by small peptide-like compounds as well as by feedback inhibition from substrate and product. A cyclic metabolic pathway for DMT has been offered. There is also evidence that DMT is taken up into synaptosomes and stored in vesicles by mechanisms identical to those described for known neurotransmitter substances. Specific binding sites for DMT have been suggested and DMT has been shown to lead to the production of CAMP, a secondary receptor messenger. As evidence of its electrophysiological activity Berridge (1972) and Berridge and Prince (1974) have shown that DMT stimulates fluid secretion from the salivary glands of blowflies, changes the transepithelial and intracellular potential of the gland, and increases the production of CAMP. Thus, DMT may fulfill the criteria for consideration as a neurotransmitter or a neuromodulator per se. Much further research is needed to elaborate this possibility. However, it does provide a basis for further speculation as to the possible mode of action of hallucinogens and perhaps a general explanation for the hallucinatory experience. There may indeed exist a hallucinogen receptor system distinct from 5-HT or DA sites. This receptor system may be the DMT system, consisting of its own presynaptic, postsynaptic, reuptake, and perhaps autoreceptor sites. As is the case with other putative transmitter systems, the DMT system may be affected by agonist or antagonist drugs or its mechanisms for synthesis, storage, uptake, and release may be altered. Thus, it is plausible that the mode of action of hallucinogens, such as LSD, may be their effect on these mechanisms, altering, perhaps, the levels of the endogenous hallucinogens, thus producing the observed effects on the psyche. Other drugs that are known to lead to the production of hallucinations may act by similar mechanisms and alterations in man’s physiological state may lead to spontaneous hallucinations, mediated through the DMT system. Only further research will lend credence to, or nullify, these hypotheses.