These results demonstrate measurable concentrations of D.M.T. in 5% of the subjects from whom plasma or whole blood samples were taken and in 16% of those on whom urinary measurements were made. Both patients with a measurable concentration of D.M.T. in plasma had an active psychotic illness; 1 of the 7 with a measurable concentration of D.M.T. in urine had an active psychotic illness. No psychiatrically normal subject in this study had a measureable concentration of D.M.T. in blood or in urine. There are several possible explanations for the infrequent occurrence of measurable blood and urinary concentrations of D.M.T. found in those patients with acute psychotic illnesses who were sampled in this study. D.M.T. may not be involved as an endogenous psychotomimetic substance or it may so function in only a few psychotic illness subtypes. It is also possible that a regionally elevated concentration of D.M.T. in the brain is the proximal event in the genesis of psychotic reactions. The appearance of detectable concentrations of D.M.T. in blood and urine will then depend on the magnitude of the quantity produced locally as well as upon the capacity of tissues to metabolise or otherwise dispose of the compound. Finally, elevations in blood D.M.T. may be episodic, in which case their detection will be critically dependent on the time of sampling. This possibility is of particular interest in view of the rapidity with which D.M.T. is known to be metabolised in man.’