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To demonstrate in the rat 9L cerebral tumor model that repeated MRI measurements can quantitate acute changes in the blood-brain distribution of Gadomer after dexamethasone administration.A total of 16 Fischer 344 rats were studied at 7T, 15 days after cerebral implantation of a 9L tumor. MRI procedures employed a T-One by Multiple Read Out Pulses (TOMROP) sequence to estimate R1 (R1 = 1/T1) at 145-second intervals before and after administration of Gadomer (Bayer), a macromolecular contrast agent (CA). Two baseline studies preceded Gadomer administration and 10 subsequent R1 maps tracked CA concentration in blood and brain for 25 minutes. Thereafter, either dexamethasone (N = 10) or normal saline (N = 6) was administered intravenously. A total of 90 minutes later a second series of 12 TOMROP measurements of Gadomer distribution was performed. The influx constant, K1, plasma distribution volume, vD, backflux constant, kb, and interstitial space, ve, were determined, and the test-retest differences of each of four vascular parameters were calculated.Dexamethasone decreased K1 approximately 60% (P = 0.02), lowered kb and vD (P = 0.03 and P < 0.01, respectively), and marginally but insignificantly decreased ve.This noninvasive MRI technique can detect drug effects on blood-brain transfer constants of CAs within two hours of administration. J. Magn. Reson. Imaging 2008;27:1430–1438. © 2008 Wiley-Liss, Inc.