N, N-Dimethyltryptamine (DMT) is a psychotomimetic substance that induces a model psychosis of brief duration when administered parentally to normal subjects. The possible endogenous formation of this compound, or a related N-methylated indoleamine, in the human body has been a recurring theme in biochemical research on the etiology of psychotic illness in man over the past decade (1, 2). One of the problems in investigating this hypothesis has been the lack of a reliable method for determining DMT in body fluids that is suitable for routine use. Existing methods either require expensive mass spectrometric facilities (3,4), lack specificity (5), or are insufficiently sensitive for many purposes (6). For instance, in the latter procedure the lower limit of detection for urinary DMT is about 0.5 pg litre-1, and this is insufficiently sensitive to determine the substance in most specimens from normal subjects. In the present paper we describe a gas chromatographic procedure for determining DMT and its precursor, N-methyltryptamine (NMT), in urine, with a lower limit of sensitivity for DMT of around 30 ng litre-1. Some preliminary results are also given on the application of this method to urine specimens from groups of control and mentally ill subjects.