High concentrations of GdDTPA in extracellular water of tissue cause the intrinsic relaxation rate of water to become greater than the coupling rate between intracellular and extracellular water compartments. The fast exchange limit is no longer valid and distinctly nonexponential spin-lattice recovery is observed. T1 relaxation recovery was characterized by a double exponential curve when striated rat muscle was immersed in a highly concentrated (110 Mm) isotonic solution of GdDTPA. When the water exchange rate through cell membranes in intact muscle tissue was calculated (using a two-compartment exchange model) and compared to similar data determined from T 2 Carr-Purcell-Meiboom-Gill experiments, a significant discrepancy was found. © 1991, American Association of Physicists in Medicine. All rights reserved.