Technical Abstract: Synephrine, the main protoalkaloid in Citrus species, is commonly analyzed as the active component in citrus peel-containing herbal supplements, but the edible parts of mandarins have been largely ignored. We determined the synephrine concentration in the juices of C. unshiu mandarins harvested from ten different groves located in a major growing region in California. For comparison, the physicochemical properties of the juices, including pH, conductivity, soluble solid content and titratable acidity were also measured. The synephrine values among ten groves ranged from 73.3 to 153.1 mg L –1. Repeat sampling of fruit from the ten locations showed that the intra-grove variability in synephrine concentrations ranged from 1.0 to 27.7%CV and were grove dependent. Among the physicochemical properties, titratable acidity weakly correlated with synephrine and for one sample a low maturity index was linked to high synephrine content. The overall mean synephrine concentration of 92.8 mg L-1 is up to six-fold higher than the values previously determined for orange juices and suggests that mandarin juice may constitute a significant dietary source of synephrine. Furthermore, the results suggest that grove location affects synephrine content.