We present here a O-17 NMR study at 1.41, 4.7, 9.4, and 14.1 T, at variable temperature and pressure of water exchange on [Ln(DTPA-BMA)(H2O)], where Ln = Nd, Eu, Tb, Dy, and Ho and DTPA-BMA = 1,7-bis[(N-methylcarbamoyl)methyl]-1,4,7-triazaheptane-1,4,7-triacetate or diethylenetriaminepentaacetate-bis(methylamide). The number of inner sphere water molecules was found from chemical shift data to be one for all complexes studied. The water exchange rate is nearly constant-(4-7) x 10(5) s(-1)-for the complexes of Nd3+, Eu3+, and Gd3+ and then takes a steep rise to reach the value of (6.0 +/- 0.3) x 10(6) s(-1) for the Ho3+ complex. This unexpectedly large change is rationalized in terms of a change of mechanism, which was determined from variable pressure studies. These show a changeover from an interchange activation mode for [Nd(DTPA-BMA)(H2O)] to a limiting dissociative mechanism for the rest of the investigated complexes : the activation volume for water exchange on [Nd(DTPA-BMA)(H2O)] is (-0.8 +/- 1.6) cm(3) mol(-1), and for the other complexes it is between +7 and +10 cm(3) mol(-1). Implications for future MRI contrast agent design are discussed : we suggest that the steric crowding at the water binding site determines the mechanism and the rate of water exchange on lanthanide(III) poly(amino carboxylate) complexes.