There is a need for new technical approaches whereby the cerebral microcirculation can be easily and continuously assessed. The objective of this study was to determine whether laser-Doppler (LD) flowmetry can be utilized to assess changes in cerebral cortical blood flow and to determine whether changes in blood perfusion measured by LD flowmetry correlate with simultaneously measured changes in flow measured by H2 clearance in cats or with changes in pial arteriolar diameter measured with a microscope in rabbits equipped with a closed cranial window. In the rabbit experiments a 0.84-mm-diam LD probe was inserted through a cranial window port, and in the cat experiments the probe was fixed adjacent to the H2 probe. The probe was fixed at a distance of 1-2 mm from the cortical surface, where it and its associated electronics detect changes in blood cell velocity and blood volume within a tissue volume of approximately 1 mm3. Volume and velocity are multiplied to provide a flow signal. When cerebral blood flow in cats was decreased by hyperventilation-induced hypocapnia and increased by norepinephrine-induced hypertension, the percent changes in LD flow and H2 clearance flow changed linearly (r = 0.94, slope = 0.97). When arterial PCO2 was increased from 28 to 48 mmHg in the rabbit experiments, the pial arterioles dilated 19 +/- 4% (mean +/- SE) and LD flow increased by 74 +/- 9%, LD flow changes which would be predicted by a third power relationship of diameter to flow.(ABSTRACT TRUNCATED AT 250 WORDS)