This review has sought to emphasise the fundamental principles on which mass spectrometric methods are based in order to indicate not only the present scope of application in medical research but also to suggest areas of future development. Studies with relevance to medical research now span the range from gas analysis to structural studies of proteins. The advent of new ionisation techniques has brought applications which stretch the capabilities of 'traditional' mass analysers (such as magnetic sector instruments) and alternative techniques (time-of-flight, ion cyclotron resonance) are finding increasing favour. Such developments represent, in general, pure research with little immediate impact on clinical practice. Another trend in instrumentation, however, is the development of relatively simple and inexpensive quadrupole mass spectrometers which give adequate performance for gas analyses and rapid GC-MS determinations of drugs or endogenous metabolites. It is likely that this divergent trend in medical mass spectrometry--both to more complex research instruments and to simpler routine instruments for the non-specialist--will continue.