The occurrence and rate of 180 degrees chain-flip motions in the crystalline regions of two polyethylenes were studied by C-13 NMR. In high-density polyethylene (HDPE) and in ultradrawn ultrahigh molecular weight polyethylene (UHMWPE) fibers, the changes in the C-13-C-13 dipolar couplings brought about by the reorientations of the C-13-C-13 internuclear vectors in the crystallites were observed. In the HDPE sample, which was labeled with 4% of C-13-C-13 pairs, the rotational motion was observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D line shape changes monitoring the C-13-C-13 dipolar coupling. The data show that the jumps occur between two sites, with a rotation angle of 180 degrees and with a jump rate of similar to 10/s at ambient temperature. The correlation function of the motion was found to be slightly nonexponential, with a stretched-exponential beta parameter of 0.8 +/- 0.1. The data yield an activation energy of 93 +/- 10 kJ/mol for the 180 degrees chain nips. In the fibers, the narrowing of natural-abundance C-13 -C-13 dipolar satellites is a clear NMR signature of the chain motion, indicating a jump rate of 150/s at 360 K, which is 20 times slower than in the unoriented HDPE. The correlation time dependence of the H-1 T-1 rho relaxation time, which probes the modulation of H-H dipolar couplings in the crystallites, was determined directly. Relations between the chain flip motion, the dynamic-mechanical alpha-relaxation, creep, and drawability are discussed.