Free fatty acids (FFAs) in human skin surface lipids were determined by high-performance liquid chromatography with electrochemical detection. The amperometric detection of FFAs was based on electrochemical reduction of quinone, 2-methyl-1,4-naphthoquinone (vitamin K(3), VK(3)) in unbuffered solution. The peak heights for lauric, myristic, palmitic, palmitoleic, stearic, oleic, linoleic, linolenic, and arachidonic acids at a detection potential of -415 mV vs a saturated calomel electrode showed a linear relationship with the amounts of acid in the range of 50-1600 pmol. Standard acids at 200 pmol were determined ten times with a relative standard deviations (RSD) of less than 1.5%. FFAs from about 1 mg of skin surface lipids absorbed on 5 x 5 mm pieces of oil-absorbing paper were extracted with 200 microl ethanol-acetonitrile (20:80) to prepare the test solution, of which 20 microl was injected into the octadecylsilica column. The results of the analysis of skin surface lipids from the forehead of a male subject were measured within 15 min of obtaining the sample. The RSD ( n=5) in all cases was less than 4.5%, and the recovery of FFAs was more than 94%. Since the size of the lipid samples was very small for the analytical method, FFAs were determined in skin surface lipids from 50 points on the head of a male volunteer and the FFA distribution on the face and scalp were mapped. The present method is noninvasive, sensitive, selective for FFAs and suitable for the determination of FFAs in skin surface lipids of humans, and has the potential to help in the diagnosis and treatment of various skin disorders.