The extracellular concentration of dopamine ([DA]e) exhibits circadian oscillations, although the mechanisms underlying this phenomenon are unclear. Through the use of voltammetry in rat brain slices, Ferris et al. showed that, over a 24-hour cycle, the rate of DA reuptake by the DA transporter (DAT) was inversely related to [DA]e. Slices from Dat-knockout mice showed no oscillation in [DA]e, indicating that diurnal variation in [DA]e is governed by the DAT.