By sampling simultaneously from two microdialysis probes placed in the left and right hindlimb somatosensory cortex, or in the somatosensory and visual or in the somatosensory and motor cortices, we compared the release of acetylcholine in functionally different regions. Samples were taken hourly from freely moving, adult male Sprague-Dawley rats for periods of 10-24h. A generalized increase in acetylcholine release occurred in all cortical regions with the transition to the night-time period of wakefulness and activity; however, the change was significantly greater in the two sensory regions (56%) than in the motor cortex (20%). Decrements in release during the active period seldom decreased the amount released below the values observed during sleep. During the active period, the amount of acetylcholine released in the somatosensory cortex was strongly correlated with the amount released in the contralateral somatosensory region and was only slightly less well correlated with the amount released in either the visual or motor cortex. The correlation between release in the somatosensory and motor cortex was not present during the day, when rats habitually sleep. These data confirm that a global change in the level of acetylcholine release occurs with a transition in behavioural state; however, because the change is not equal in all areas and, because the correlation between motor and sensory cortex can be uncoupled, it seems likely that there are additional mechanisms available for independent control of acetylcholine release within specific cortical regions.