Rare-earth-doped fibers, such as Er 3+ - and Yb 3+ -doped aluminosilicates can be adva ntageous in space-based systems due to their stability, their high-bandwidth transmission properties and their lightweight, small-volume properties. In such environments the effect of ionizing-radiation on the optical transmission of these fibers is of paramount importance. For the present work, gamma-radiation experiments were conducted in which un-pumped Yb 3+ and Er 3+ doped sample fibers were irradiated with a Cobalt-60 source under different dose-rate and temperature conditions. In-situ spectral transmittance data over the near IR was monitored during the irradi ations for total doses of up to tens of krad (Si). It was found that there was a dose-rate dependence in which higher rates resulted in more photodarkening. Higher temperatures were not found to significantly affect the rate of photodarkening at the dose rates used. Keywords: Gamma-radiation effects, photodarkening, radiatio n-induced absorption, dose-r ate dependence, dose-rate effects, temperature effects, Co-60 ir radiation, rare-earth doped fibers