This work is a sequence of a previous one where we investigated the influence of gamma radiation dose on environmental stress cracking (ESC) of a conventional poly(methyl methacrylate) (PMMA) (Sousa et al. Polym Degrad Stab 92:1465, 2007). In that work, we observed that low doses of gamma radiation intensified the ESC effects, but on higher doses the gamma radiation effect was predominant over the stress cracking. The present work describes a similar study, but using a toughened PMMA (t-PMMA). The polymer was submitted to gamma radiation doses up to 100 kGy, and then tested to ESC susceptibility through tensile and relaxation tests. Two different types of active fluids were used: ethanol (an aggressive one) and ethylene glycol (a moderate liquid stress cracking agent), based on absorption results. A synergistic effect between ESC and radiation degradation was noted, with a substantial decay in mechanical properties when these two effects were present. The ethanol action caused dendritic pattern formation in the fracture surfaces of t-PMMA, as revealed by scanning electron microscopy.