Nuclear magnetic resonance (NMR) spin-lattice relaxation times (T-1) in various polyethylene and polypropylene resins were measured at 20 MHz and at temperatures of 130-490 K. At each temperature and for all resins, only a single value of T-1 was found, which was consistent with the occurrence of rapid spin diffusion throughout the protons attached to the polymer chains. The data were analyzed for the estimation of activation energies corresponding to molecular motion causing spin-lattice relaxation. Two well-defined minima were found for log(c)(T-1) plotted as a function of temperature for all of the polypropylene resins. Single very broad minima were found for all of the polyethylene samples. In contrast, the free induction decay signals from all of the resins following single radio-frequency pulses were observed to contain a rapidly decaying component followed by a much more slowly decaying signal. These components were used to estimate the amount of rigid component present in the solid resins at room temperature. Samples of one high-density polyethylene and one low-density polyethylene were irradiated with gamma radiation up to a 500-kGy dose to examine the effects of crosslinking on the NMR relaxation.