The responsiveness of cerebral arteries to the changes in arterial carbon dioxide tension (paCO2) was studied in spontaneously hypertensive rats (SHRs) and normotensive rats (NTRs). A freeze substitution method was applied for the preparation of pial and cortical arteries for morphometrical study. Hypercapnia was induced by giving 8% CO2, and hypocapnia was provided by hyperventilation. The ratios of internal (d) to external diameter (D) (d/D ratio) of both pial and cortical arteries in SHR were not different from those in NTRs during normocapnia. In hypercapnia, the ratios of pial and larger cortical arteries (D greater than or equal to 20 micron) in SHRs were 80.9 +/- 0.8% and 78.6 +/- 0.6%, respectively, being significantly smaller than 86.2 +/- 0.7% and 82.2 +/- 0.5% in NTR. In contrast, the d/D ratio of pial arteries in hypocapnia was 72.5 +/- 1.4% in SHRs, which was significantly larger than 67.5 +/- 1.4% in NTRs. The responsiveness of smaller cortical arteries (D less than 20 micron) to paCO2 was not different between SHRs and NTRs. The present results suggest that in SHRs cerebrovascular CO2 reactivity is decreased as compared to NTRs, especially in pial and larger cortical arteries.