Molecular fluorescence is much in use nowadays as an analytical technique, particularly for the determination of traces of toxic fluorescent substances. The authors describe a spectrofluorimetric method, called the synchronous excitation method, which is more sensitive and more selective than the classical spectrofluorimetry and which can be used for the direct determination, in the presence of other hallucinogens, of LSD in confiscated samples. They also propose the application, under certain conditions, of the method as a rapid test for the presence of LSD.