In most European countries 5-FU is given as an infusion during 46 hours in combination with a two hour infusion of LV. This regime is called the deGramont schedule. In the Nordic countries, 5-FU and LV are often given as bolus injections according to the regime called Nordic FLV. Leucovorin is a prodrug which has to be metabolized to methylenetetrahydrofolate (mTHF) in order to increase inhibition of the target enzyme thymidylate synthase (TS). Due to the low concentrations of the reduced folate forms intracellular there is a lack of knowledge of the pharmacodynamics of LV when used as an infusion. When the 5-FU/LV used today was introduced it was technically not possible to measure the different metabolites of the reduced folates in blood and tissue. In the literature there is only one published reference measuring the blood concentration after a two hour infusion of LV. However, it has recently been possible to separate and quantitate the different folates using sensitive Liquid Chromatography/Masspectrometry (LC/MS) methods. Furthermore it has been stated that differences in enzyme activity for genes involved in the transport and metabolism of folates may be responsible for the large inter individual differences in folate tissue concentrations.