Laboratory observations have noted that during normocapnia and intact vascular tone, changes in the intracranial pressure (ICP) and arterial blood pressure (ABP) recordings are not similar. The purpose of the present study was to determine whether: 1) the diameter of cerebral pial arterioles synchronously increases during positive pressure inhalation and decreases during the expiration phase of ventilation; and 2) the variation in arteriolar diameter is greater when vascular tone is intact than when arterioles are maximally dilated. Severe hypercapnia was induced by prolonged ventilation with gas mixture of 20% O2, 10% CO2, and 70% N2 in 8 piglets. S-VHS recordings of 3600X magnification of nine pial arterioles were obtained during conditions of normal vascular tone and severe hypercapnia and digitally analyzed by an image-averaging method. Variation of the amplitude of arteriolar diameter over the ventilation cycle was computed and found to synchronously increase during positive pressure inhalation and to decrease to a steady state baseline during passive expiration. Mean amplitude of variation in diameter (+/- S.D.) during normal tone and hypercapnia was computed as 1.63 (+/- 0.693) microns and 1.012 (+/- 0.869) microns respectively and found to be significantly greater during the condition of normal vascular tone than during hypercapnia (p < .006). These results suggest that a cyclic variation of arteriolar blood volume is one causal factor related to the low frequency variation of the baseline of ICP associated with positive pressure ventilation.