The pulsed-laser polymerization method was applied to determine the propagation rate constant k(p) of N-vinylindole (VI), and the Arrhenius parameters were evaluated in the range of 30-70 degreesC. One of the characteristics of the VI monomer was its low activation energy (17.5 kJ mol(-1)) compared with other vinyl monomers, indicating that the propagating radical has a high reactivity. Although the reaction rate constant k(p) = 86 L mol(-1) s(-1) at 30 degreesC was relatively small and similar to that of styrene, the Q-value and the products of photochemical reactions suggested again that it has a higher reactivity than styrene monomer. This property of VI radical probably gives rise to high termination probability and the cyclic structure at the chain end of the linear polymer.