THE sweet odour of Cannabis sativa, L. is of special interest as a possible means for the detection of illicit marijuana traffic. US Customs dogs, trained to alert to the odour of marijuana and hashish, are successfully used to find concealed contraband. An electronic “sniffer” that employs a portable quadrupole mass spectrometer to detect volatile vapours of drugs, including marijuana, has also been demonstrated1. Although the chemical composition of the essential oil of marijuana has been thoroughly investigated2–4, little is known about the specific composition of the emitted aroma constituents. The composition of this headspace, as it contributes directly to the odour, is more significant than that of the oil for the characterization of marijuana aroma5. Aroma profiles of fruits and vegetables have been obtained by direct gas chromatographic analysis of the headspace components6. We used this technique, which permits rapid determination of labile constituents under mild analytical conditions, to analyse marijuana aroma, and we report our preliminary results here.