This article describes a simplified method for the determination of the L-[18F]6-fluorodopa (FDOPA) fraction time course that takes advantage of the strong correlation between the radioactivity ratio (metabolites/FDOPA) and time. Serial arterial blood samples are collected for assay of plasma total radioactivities following an intravenous injection of FDOPA into carbidopapretreated subjects. In addition, a single plasma sample, collected late in the study and analyzed for FDOPA fraction, is sufficient to determine accurately the time course of the FDOPA concentration in plasma. The validated straight-line method greatly simplifies blood analysis for routine positron emission tomography FDOPA studies.