Cyclic voltammetry has been used to study the reduction of phylloquinone (2-methyl-3-phytyl-1,4-naphthoquinone)(vitamin K1) at a planar glassy-carbon electrode. The reaction was found to be quasi-reversible with adsorption of the product, but not the reactant, taking place at the electrode surface. Liquid chromatography with electrochemical detection was used to determine endogenous levels of phylloquinone in human plasma. The normal range was found to be 0.08–1.24 ng ml–1; the mean value was 0.41 ng ml–1. The electrochemical assay was shown to be about three times as sensitive as the previous liquid chromatographic method in which the naphthoquinone was detected by its ultraviolet absorption. Agreement between the two methods was good when phylloquinone was added to plasma in the range 0–4.8 ng ml–1(r= 0.91; 0.001 < p <0.01).