In perfusion experiments, previous investigations have shown an increased responsiveness of isolated vascular beds from spontaneously hypertensive rats (SHRs) compared with Wistar-Kyoto controls (WKYs). The results have suggested that there are differences in the arterial vessels but not in the venous vessels. Here we have tested the latter part of this hypothesis by comparing the contractile and morphological properties of isolated portal veins from 18 SHRs and 18 WKYs, all 3-4 months old. We found no differences between these veins as regards: (a) maximum force response; (b) active and passive force--length relationship; (c) sensitivity to exogenous noradrenaline without and with inhibition of neuronal uptake by cocaine, and (d) cross-sectional area of the longitudinal smooth muscle layer. There was, however, evidence for difference at the cellular level, for the active force per unit cell area was 17% lower in the SHR vessels compared with that in the WKY vessels. With this exception, the results indicate that the morphological and functional differences which have been reported by others for arterial vessels from SHRs and WKYs do not prevail in the portal vein taken from rats at 3-4 months of age.