The basal forebrain (BF) is a heterogeneous structure located in the ventral aspect of the cerebral hemispheres. It contains cholinergic as well as different types of noncholinergic corticopetal neurons and interneurons, including GABAergic and peptidergic cells. The BF constitutes an extrathalamic route to the cortex, and its activity is associated with an increase in cortical release of the neurotransmitter acetylcholine, concomitant with electroencephalographic (EEG) low-voltage fast activity (LVFA). However, the specific role of the different BF cell types has largely remained unknown due to the lack of chemical identification of the recorded neurons. Here we show that the firing rate of immunocytochemically identified cholinergic and parvalbumin-containing neurons increase during cortical LVFA. In contrast, increased neuropeptide Y neuron firing is accompanied by cortical slow waves. Our results, furthermore, indicate that BF neurons posses a distinct temporal relationship to different EEG patterns and suggest a more dynamic interplay within BF as well as between BF and cortical circuitries than previously proposed.