AbstractThe rotational relaxation times of rabbit myosin and myosin rod have been determined by electric birefringence measurement. The relaxation time of myosin measured in 10 mM pyrophosphate buffers in a pH range of 7.6–9.5 was found to have substantial concentration and pH dependences. The infinite‐dilution limit of the relaxation time, τ°, was determined as 38 ± 2 μs, and it was found to be independent of pH. For myosin rod, a possible thermally induced conformational change was investigated in a temperature range of 1–43°C. The rotational relaxation time of myosin rod shows no clear indication of conformational change in this temperature range, and the radius of gyration measurement by light scattering was shown to be consistent with this observation. The steady‐state birefringence, however, decreases substantially above around 40°C. This, the myosin rod appears to be only slightly flexible even at physiological temperature, but the possibility of a “melting” or “hinging” of the myosin rod cannot completely be ruled out on the basis of these experiments.