To examine the role played by the basal forebrain cholinergic system in cortical activation, neuronal activity was investigated in the globus pallidus and substantia innominata of urethane-anaesthetized rats during large cortical slow waves and spontaneous or elicited low voltage fast activity. An effort was made to identify the neurones by antidromic stimulation from the neocortex (supposedly cholinergic cells) and from the subthalamic nucleus (pallidal cells). Most of the cortically projecting neurones were strongly activated during cortical activation (5-fold increase in firing rate on average), while the discharge rate of pallidal units was increased only slightly (ratio 1.25 on average). In contrast to the cortically projecting cells, some unidentified cells in the substantia innominata fired at a much higher rate during large cortical slow waves as compared to low voltage fast activity. The results are discussed in relation to previous work on the cortically projecting cells and on the mechanisms of cortical activation.