Reports of behavioral and electrophysiological changes in response to low-level UHF-microwave irradiation are not easily reconciled with known biophysical mechanisms. Effects at incident power densities on the order of 10 mW/cm 2 or less would imply either enhanced sensitivity of the nervous system to induced thermal loads or the existence of stronger nonthermal modes of interaction than hitherto proposed. In this paper, a hypothesis is developed that accounts for a class of such reportedly nonthermal effects. It is proposed that absorbed electromagnetic (EM) radiation yields thermal gradients within the semicircular canals of the labyrinth. The intravestibular convective torque induced thereby would mimic natural vestibular simulation. Detection and nystagmus thresholds with corresponding response latencies for human adults and for monkeys are derived in terms of incident power density. For the former, the detection threshold is estimated to be 35 mW/cm 2 ; the threshold may be significantly lower when the possibility of resonance absorption exists, as with cranial radii on the order of 5 cm or less (immature human or small animal). These results imply the existence of a significant caloric vestibular cue at incident power densities that are not otherwise associated with gross thermal effects.