For a bulk GaAs semiconductor laser amplifier the evolution of the optical gain after a 200 fs pulse excitation is calculated by including carrier–carrier and LO‐phonon–carrier scattering in terms of the Boltzmann collision integrals. For an injected light pulse in the gain (absorption) region the optical gain shows a transient decrease (increase) followed by relaxation towards a lower (higher) level. Around the transparency point the broadening of the optical transition due to the decay of the induced interband polarization yields a transient decrease of the optical gain although the carrier density does not change. This behavior agrees qualitatively well with measurements of M. P. Kesler and E. P. Ippen [Appl. Phys. Lett.51, 1765 (1987)]. The temperature relaxation after the excitation is examined. ©1995 American Institute of Physics.