Immobility reflex evoked by vertical lifting of the rat.

It is assumed that the immobility reflex (IR) evoked in adult rats by sensory stimulation does not exceed a few seconds. However, we easily achieved a longer lasting (up to 3 min or longer) immobility state in most of the tested rats when they were grasped by hand and lifted to a specific upright position. Twenty male rats originating from two different albino populations could remain immobilized continuously in such a position for average durations of 138 and 48 s respectively, while in 20 hooded male rats and 20 albino female rats the mean immobility durations were 138 s and 36 s, respectively. It is suggested that IR evoked in rats is homologous with similar reflexes in other species, the assumption that the rat is not useful for IR studies deserts reconsideration.