Small signal gain measurements of optically excited terahertz silicon lasers are reported. Two types of lasers, Si:P and Si:Bi, were investigated. They were optically excited with radiation from a free electron laser or a CO2 laser. The experiments were performed with an oscillator-amplifier scheme where one sample serves as a laser while the other one is an amplifier. In case of the free electron laser the pump frequency corresponds to intracenter excitation of the 2p0 or 2p± states of the P and Bi Coulomb centers, and the gain was determined for the 2p0?1s(E), 2p0?1s(T2) transitions in Si:P and the 2p±?1s(E) transition in Si:Bi. Pumping with a CO2 laser leads to photoexcitation of the Coulomb centers. In this case the gain was determined for the 2p0?1s(T2) of Si:P transition. The gain for intracenter pumping is in the range 5?10?cm?1 while for photoexcitation the gain is considerably less, namely ? 0.5?cm?1. The experimental results are analyzed and found to be in good agreement with theoretical calculations based on balance equations.