Technical Abstract: Nobiletin (NOB) and tangeretin (TAN), two of the main polymethoxylated flavones in citrus, influence a number of key biological pathways in mammalian cells. While the impacts of NOB and TAN on glucose homeostasis and cholesterol regulation have been investigated in human clinical trials, much information is still lacking about the metabolism and oral bioavailability of these compounds in animals. In this study, NOB and TAN were administered to rats by gavage and intraperitoneal (IP) injection, and the blood serum concentrations of these compounds and their main metabolites were monitored by high performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS). In addition to the original compounds, two metabolites of TAN and 8 metabolites of NOB were detected and monitored. Nearly 10 fold higher absorption of NOB occurred following oral administration compared to TAN. For both compounds, maximum levels of glucuronidated metabolites occurred in the blood serum at later time points (~5-8 h) compared to the earlier Tmax values for NOB and TAN. In most cases the glucuronides occurred at substantially higher levels than those of aglycone metabolites. Low levels of NOB and TAN and their metabolites were detectable in serum even at 24 h post treatment. Eight of the metabolites detected in these studies were isolated and preliminarily analyzed by UV, FTIR, ESI-MS and NMR.