The rates for the 1,2-sigmatropic hydrogen and deuterium shifts in the ground state of the photorearranged intermediate of N-acetylpyrrole were directly measured by means of laser flash photolysis in several solvents; (e.g., 0.27 s(-1) for 1,2-H shift and 0.12 s(-1) for 1,2-D shift in nonpolar methylcyclohexane (MCH) at 293 K). The rate of the 1,2-hydrogen shift was remarkably increased by a basic catalyst, such as triethylamine, alcohols, and water. From the experimental results of temperature and isotope effects, it was shown that the 1,2-sigmatropic hydrogen (or deuterium) shift in MCH proceeds via quantum mechanical tunneling processes at two vibrational energy levels : E = 0 (upsilon = upsilon(0)) and E = E(v) (=2.9 kcal mol(-1) for the hydrogen shift or 3.3 kcal mol(-1) for the deuterium shift) (upsilon = upsilon(1)) under experimental conditions. The theoretical considerations for the tunneling mechanism were made by use of the tunnel effect theory proposed by Formosinho. The rates obtained by theoretical calculations were in good agreement with experimental ones. It is noteworthy that the 1,2-sigmatropic hydrogen (or deuterium) shift takes place via the intramolecular process at a low concentration of N-acetylpyrrole (1.7 x 10(-4) M) in dehydrated MCH.