A method for detecting trace quantities of organic drug material in a living animal under observation wherein samples of gaseous material effusing from the animal either by way of breath, through the skin or from a blood stream are passed through a three-stage membrane gas separator to enrich the concentration of the organic drug material in the gaseous sample. The enriched sample gas output of the membrane separator is fed to a gas analyzer such as a mass spectrometer for detecting the organic drug material, if any, in the sample gas under analysis. The detection of trace quantities of drugs may be employed for monitoring the rate of metabolism and for ascertaining the various metabolites produced by metabolism of drugs in the body.