The midbrain periaqueductal gray matter has been identified as a reflex centre located uppermost in the central organization of diverse defensive reactions. We recently found that when activated, the caudal third of the lateral periaqueductal gray was also capable of provoking a marked increase in cortical blood flow. The response may be the combined outcome of a flow increase of nitrergic origin and that coupled to a possible concomitant cortical activation. In the present study, we attempted to clarify the neural substrates for mediation of the increase in flow (observed by laser-Doppler flowmetry), in 49 anaesthetized, artificially ventilated, and cervically cordotomized rats. The flow increase provoked by stimulation of the particular subdivision of the periaqueductal gray with N-methyl-D-aspartate (1 mM, 100 nl) was unaffected by i.v. pentolinium tartrate (10 mg/kg), suggesting little contribution by the cerebrovasodilator parasympathetic nervous system to the response. The response was abolished by i.v. or topical cortical administration of scopolamine hydrobromide (3.16 mg/kg or 1.0 mM, respectively). Placement of bilateral lesions in the basal forebrain with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (15 mM) impaired the cortical choline acetyltransferase activity and attenuated the flow response. Overall, we suggest that the cholinergic corticopetal neurons of the nucleus basalis of Meynert and cortical muscarinic receptors may form a principal efferent arm of a central circuitry emanating from the subdivision of the periaqueductal gray, in the mediation of the increase in cortical blood flow and possible cortical activation.