We describe a method of measuring spin-lattice relaxation rates of dilute nuclei using optical hole burning. This was applied to Pr141 nuclei doped into LaF3 at low concentrations (0.01 to 2.0 at.%). The observed relaxation rates varied by about two orders of magnitude between 2 and 4.5 K. At the lowest concentrations the data are interpreted in terms of two mechanisms: resonant two-phonon (Orbach) relaxation with a 57-cm-1 activation, and praseodymiumlanthanum cross relaxation. At higher concentrations an additional hole-filling process was observed and attributed to homonuclear Pr-Pr dipolar coupling. Theoretical estimates of the phonon-induced relaxation rates are given and agree approximately with experimental values. © 1980 The American Physical Society.