In a rat experiment (n=59), a single 40-minute whole-body 28 GHz exposure at near-threshold WBA-SAR levels was evaluated under normal and heat conditions with restraint. After accounting for sham-related restraint stress, exposure was associated with increased serum-free corticosterone 1–3 days later, especially when rectal temperature rose by >1°C. Urinary catecholamines suggested an immediate inhibitory effect on stress response (notably noradrenaline), with heat amplifying effects and linking noradrenaline to tail surface temperature.