The electron-transfer rate constant is measured in two U-shaped donor-bridge-acceptor molecules over a wide range of temperature in acetonitrile and N-methylacetamide (NMA). The electron-transfer rate at high temperature can be well described by a nonadiabatic model of the reaction, but at low temperatures the rate in NMA becomes controlled by the solvent. The results are discussed in terms of theoretical models for the change in reaction mechanism and its dependence on the solute-solvent frictional coupling.