The two-photon transition probability rate constant for the transition from the ground state (S-1(e)) to the D-1(e) excited state of argon has been calculated by the means of multiconfiguration self-consistent field quadratic response methods. The rate constant for the D-1(e) transition in the Hartree-Fock approximation is 0.119x10(-50) cm(4) s, while in the multiconfiguration Hartree-Fock calculation it is 0.191x10(-50) cm(4) s. The calculated rate constant is in an excellent agreement with the experimental rate constant of 0.205x10(-50) cm(4) s obtained as the sum of the rate constants for the transitions to the spin-orbit mixed 3p(5)4p[3/2](J=2) (D-1(e)), 3p(5)4p[5/2](J=2) (D-3(e)), and 3p(5)4p’[3/2](J=2) ((3)p(e)) states.