The 13C NMR of solid poly(Me methacrylate (I) [9011-14-7] was detd. by cross-polarization magic-angle spinning to detn. the effects of tacticity and plasticization on spin-lattice relaxation rates. Isotactic [25188-98-1], atactic, and syndiotactic I [25188-97-0] and 1:2 isotactic-syndiotactic I complex [79616-50-5] were studied with 2,4,8,10-tetraoxaspiro[5.5]undecane [126-54-5] or Me methacrylate [80-62-6] plasticizers. Reorientation of a-Me protons is the dominant relaxation mode for CH2, a-Me, and quaternary C nuclei. Relaxation is slower in isotactic than in syndiotactic I, owing to faster reorientation of the a-Me group in the former. The increase of chain mobility by plasticization had no detectable effect on relaxation rates.