The excretion of N,N,-dimethlytryptamine (DMT) has been measured in longitudinal studies of five patients with schizo phrenic illnesses and in four patients with rapidly or slowly cycling manic-depressive illness. The excretion of DMT was frequently raised in patients when they were psychotic but was usually normal when they had recovered. However, rapid changes in the severity of illness or sudden switches from one mood state to another were not accompanied by changes in the excretion of DMT. These findings contrast with the immediate hallucinogenic effects of an injection of DMT, and suggest that the extracerebral production of DMT (as measured by its urinary excretion) does not provoke the experience of hallucinations in psych otic patients.