Though melatonin is primarily metabolised to 6-hydroxy-melatonin, we have recently shown that it can also be demethylated to form N-acetyl-serotonin. The question therefore arises as to whether demethylation is a general metabolic pathway that can apply to other pineal methoxyindoles. To investigate this possibility we administered deuterated methoxy-tryptophol (dML) and deuterated methoxy-tryptamine (dMT) to rats and analysed the urine for the presence of deuterated methoxyindole acetic acid (dMIAA) and deuterated hydroxyindole acetic acid (dHIAA). The method of analysis was gas chromatography mass spectrometry (GCMS), where the relevant molecular ion and fragment ions were monitored. The results showed that the major metabolite in all cases was dMIAA. There was no evidence to suggest that the compounds had been demethylated to form dHIAA. The study therefore indicates that the demethylation of melatonin is a specific metabolic pathway that does not apply to other methoxyindoles.