The reorientational relaxation process of the photochromic molecule 9-hydroxymethyl-10-[(naphthylmethoxy)-methyl]anthracene (HNMA) was observed in glassy poly(methyl methacrylate) (PMMA). For this purpose, the intramolecular photodimerization of HNMA incorporated in a PMMA matrix was selectively induced by irradiation with linearly polarized light. The optical anisotropy created in this way in the sample was allowed to relax to equilibrium by annealing at different temperatures. The reorientational relaxation of HNMA associated with this thermal treatment was subsequently observed by monitoring the time dependence of the two dichroic absorbance components of the anthracene moieties of HNMA. It was found that this relaxation process is well described by the sum of an exponential function of annealing time and a baseline which strongly depends on temperature. This fact implies that HNMA was not completely randomized in the glassy state of PMMA upon annealing. Furthermore, within the range of experimental temperatures, the activation energy of the reorientational relaxation rates of HNMA is close to that of the gamma-process of the PMMA matrix, suggesting that the relaxation of the anthracene moieties is affected by the rotational motions of the alpha-methyl groups on the PMMA chain backbone. These preliminary results might provide some helpful guiding principles for controlling the stability of host/guest polymer materials using non-linear optical dyes.