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Department of Physiology, School of Medicine, Medical School, Edgbaston, Birmingham, UK.

1. Direct observations were made of responses to systemic hypoxia (breathing 12 or 6% O2 for 3 min) evoked in terminal arterioles (TA, 14-30 microns internal diameter), precapillary arterioles (PCA, 8-18 microns), collecting venules (CV, 12-30 microns) and small veins (SV, 20-50 microns) of the mesenteric circulation of the anaesthetized rat. Changes in vessel diameter were recorded before and after local blockade of alpha-adrenoreceptors with phentolamine when the mesentery was covered with Saran Wrap, which is impermeable to O2, and then after removal of the Saran Wrap, which would have kept local PO2 relatively high even during systemic hypoxia. 2. The majority of TA showed an initial decrease in diameter of 14 +/- 1% (mean +/- S.E.M.). These responses were reversed to increases in diameter (12 +/- 2%) after phentolamine, but virtually abolished after removal of the Saran Wrap (0.3 +/- 2%). 3. Some PCA showed similar behaviour to the TA; others showed an increase in diameter (11 +/- 1%). The increases in diameter were accentuated after phentolamine (16 +/- 1%), but were reduced after removal of the Saran Wrap (6 +/- 2%). 4. CV and SV showed either a decrease in diameter followed by relaxation towards control levels, or an increase in diameter that waned before hypoxia ceased (6 +/- 1% and 1 +/- 1%, respectively). The responses of CV were not altered by phentolamine (8 +/- 1%), but SV showed larger increases in diameter (5 +/- 1%).(ABSTRACT TRUNCATED AT 250 WORDS)