Temporal changes in theT1 andT2 relaxation rates (ΔR1 and ΔR2) in rat olfactory bulb (OB) and cortex were compared with the absolute manganese (Mn) concentrations from the corresponding excised tissue samples. In vivoT1 andT2 relaxation times were measured before, and at 1, 7, 28, and 35 d after intravenous infusion of 176 mg/kg MnCl2. The values of ΔR1, ΔR2, and absolute Mn concentration peaked at day 1 and then declined to near control levels after 28 to 35 d. The Mn bioelimination rate from the rat brain was significantly faster than that reported using radioisotope techniques. TheR1 andR2 relaxation rates were linearly proportional to the underlying tissue Mn concentration and reflect the total absolute amount of Mn present in the tissue. The in vivo Mnr1 andr2 tissue relaxivities were comparable to the in vitro values for aqueous Mn2+. These results demonstrate that loss of manganese-enhanced MRI (MEMRI) contrast after systemic Mn2+ administration is due to elimination of Mn2+ from the brain. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.