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The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature interval (5–290 K). The complex nonexponential recovery of the longitudinal magnetization in this temperature interval has been described with two characteristic relaxation times, 1/T*_1a as the faster component and 1/T*_1b as the slower component, respectively. It was shown that 1/T*_1a is strongly affected by the CH₃ group dynamics of the SAR1 center. The complete temperature dependence of 1/T*_1a was described by invoking several relaxation mechanisms that involve hindered motion of the CH₃ group from classical rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (ΔE/k=1570 K). On the other hand the temperature dependence of 1/T*_1b is …