Intracellular recordings were obtained from thalamic nuclei neurones in isolated and perfused whole brains of adult guinea pigs in vitro. Thalamic neurones presented the same basic membrane properties in this preparation as in slices. The majority of cells displayed, in addition, spontaneous spindle oscillations similar to those observed in the cat in vivo. This activity could be blocked by the cholinergic agonist carbachol perfused at 10(-4) M. Thalamic spindles could also be recorded in decorticated preparations, in which perfusion to the cerebral cortex was interrupted. Such preparations could be useful for studying in vitro the spindle generating mechanism in rodents as well as some aspects of brainstem/diencephalic interactions related to the control of sleep and arousal.