A proton dynamic nuclear polarization (DNP) NMR signal enhancement (⑀) close to thermal equilibrium, ⑀ ؍ 0.89, has been obtained at high field (B 0 ؍ 5 T, epr ؍ 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spinlock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T 1 ), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T 1n ). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T 1 and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T 1n ). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time 1/2 of ⑀ t ؍ 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for highfield DNP/NMR studies of biological systems.