Rate constants of polymerization of L-proline NCA initiated by n-butylamine in acetonitrile and benzene were determined by measuring the CO₂ evolved in the reaction. Although the reaction systems were heterogeneous (both acetonitrile and benzene were nonsolvent for poly-L-proline), the rate of polymerization could be represented as second order rate equation which was first order with respect to monomer and initiator concentrations respectively. The polymerization rate of L-proline NCA proceeded fast at the early stage of polymerization(up to about 30 minutes) but considerably slow in the ensuing stage of polymerization. The reaction rate constants were obtained at temperatures ranging from 0℃ to 35℃. The activation energies for fast and slow polymerization, E_f and E_s were determined; in acetonitrile, E_f=8.1kcal/mole and E_s=2.2kcal/mole, and in benzene E''_f=5.3kcal/mole and E''_s=2.5kcal/mole, respectively. The polymerization rate of L-proline NCA in benzene showed lower transition point than that in acetonitrile at low degree of polymerization. It was postulated that the structures of polymers obtained in acetonitrile and benzene solvents might be different.