The hallucinogenic substance N′,N′-dimethyltryptamine and its precursor N-methyltryptamine were found in 24-h speciments of urine from 19 normal human subjects; the mean excretion rates were 386 ng 24 h-1 and 856 ng 24 h-1 respectively. The urinary excretion of both compounds was unrelated to age, sex, urinary volume, or creatinine, nor was any consistent diurnal pattern observed. Rates for the mono and dimethylated compounds were not correlated. Diet and the intestinal flora were excluded as a source of urinary dimethyltryptamine. Administration to 4 subjects of sufficient ammonium chloride to increase the H ion concentration of the urine caused a transient increase in dimethyltryptamine excretion but no consistent increase in the rate for N-methyltryptamine. Acidification of the urine did not appear to be the determining factor in this result since in one subject the same drop in urinary pH was achieved by feeding methionine without any increase in dimethyltryptamine excretion.