Local cerebral perfusion pressure (CPP), a crucial parameter that should allow a better assessment of the haemodynamic compromise in cerebrovascular diseases, is not currently measurable by non-invasive means. Experimental and clinical studies have suggested that the regional ratio of cerebral blood flow to cerebral blood volume (CBF:CBV), as measured by PET, represents an index of local CPP in focal ischaemia. The present study was designed to evaluate further the reliability of the CBF:CBV ratio during manipulations of CPP by deliberately varying mean arterial pressure (MAP) in the anaesthetized baboon. Cortical CBF, CBV, cerebral metabolic rate for oxygen (CMRO2) and oxygen extraction fraction were measured by PET using the (15)O steady-state technique in 10 anaesthetized baboons. Five baboons (Group A) underwent four PET examinations at different levels of MAP: base line (101 +/- 6 mmHg) followed by moderate hypotension (58 +/- 3 mmHg) and, in a separate experiment, minor hypotension (72 +/- 3 mmHg) followed by profound hypotension (34 +/- 5 mmHg). Trimetaphan was used to lower MAP to minor and moderate levels while profound hypotension was achieved by the combined effects of trimetaphan and lower-body negative pressure. Five other baboons (Group B) were subjected to hypertension (121 +/- 2 mmHg) induced by metaraminol and were compared with their base line state (81 +/- 10 mmHg). While CBF displayed significant changes with varying MAP, i.e. decrease and increase with hypotension and hypertension, respectively (-11% from base line to moderate hypotension compared with -20%, from minor to profound hypotension and +31% from base line to hypertension), CBV was more variable and did not significantly change, except with profound hypotension when the increase was significant (+13%). The CBF:CBV ratio decreased significantly at all stages of hypotension (-21 and -31%) and was significantly increased during hypertension (+30%). Importantly, the CBF:CBV ratio demonstrated a significant correlation with MAP (rho = 0.78, Spearman's rank correlation coefficient, P < 0.01). No major changes in CMRO2 were noted during either hypotension or hypertension. Our results demonstrate that, under physiological conditions, cortical CBF:CBV is significantly correlated with CPP, itself a function of MAP. In the investigated range of MAP, the relationships between CBF:CBV and MAP appear to be linear. These findings further argue for the reliability of CBF:CBV as an index of CPP in situations where increases or decreases of MAP without superimposed changes in cerebrovascular tone are encountered, and they confirm the potential usefulness of this regional ratio for clinical investigations and management in cerebrovascular diseases.