It is shown that the production of field desorption mass spectra with an emission-controlled emitter heating device is a useful tool in obtaining information about molecular structures. Since the structurally significant signals are sometimes missing in the corresponding electron impact spectra but not in the field desorption spectra the use of field desorption mass spectrometry appears to be more advantages. This will be especially true for substances that are thermally labile, unvolatile or unstable upon electron impact (both at high and low electron energy), the latter being the case for some of the compounds presently investigated which are derivatives of a drug with antitumour and antiepileptic effects. Further, emission-controlled desorption improves the reproducibility of the spectra obtained and thus appears to be a prerequisite for quantification in biomedical and pharmacokinetic studies.