The vasoconstrictor effects of noradrenaline were studied in spontaneously hypertensive rats (SHR) compared with Wistar Kyoto rats (WKY), and in Wistar rats with regional hypotension (WH) compared to control Wistar rats (WC). The abdominal aorta was ligated in WH distal to the renal arteries, lowering blood pressure in the hindquarters by 41% and tail artery wall cross-sectional area by 35% compared with WC. A cylindrical segment was dissected from the proximal part of the tail artery, cannulated at both ends and perfused with Krebs-Henseleit solution either at constant flow starting from a pressure of 120 mmHg or at a constant pressure of 120 mmHg. The cumulative dose± response relationships for noradrenaline were determined in control conditions and subsequently in the presence of gadolinium (100 lM), a non-speci®c blocker of mechanosensitive channels. Under constant-¯ow perfusion noradrenaline evoked a more prominent resistance increase in SHR compared with WKY and in WC compared with WH. Similar relations were seen in the presence of gadolinium, although responses were reduced. At constant pressure perfusion the vasoconstrictor response to noradrenaline was lower in SHR compared with WKY and in WC compared with WH. Application of gadolinium under constant-pressure perfusion reduced responses in WKY and WH, so that vasoconstriction in SHR became more pronounced than that in WKY and in WC than that in WH. It is suggested that the results can be explained by the difference in wall thickness causing different degrees of activation of the myogenic response to distension.