The tricyclic antidepressant group of drugs are amongst the most widely used medicaments in current clinical practice. They have proved to be extremely useful in the treatment of many different types of depression. There are, however, greater than ten-fold differences between different individual subjects in the rate at which they can degrade the various members of the group into biologically inert metabolites. There is also good evidence that the therapeutic effectiveness of this class of drug is more closely related to thtr go-called 'steady state' plasma concentrations than to the dose administered . It is clearly important, therefore, to be able to measure plasma drug concentrations simply and rapidly. Methods currently available include luorimetry, thin layer chromatography (TLC), gas liquid chromatography (GLC), high pressure liquid chromatography (HPLC) and isotope derivatization, but are generally too insensitive, non-specific or slow to be useful clinically.