The effect of intraluminal hyperglycaemia was investigated in the iliac artery of 11 anaesthetised pigs. Following isolation of a test segment, hyperglycaemic blood (40 mmol.L-1) caused a significant dilatation of the artery of 167 +/- 208 mu m (mean +/- SD; n = 6, P = 0.031). Dilatations were reduced by N(G)-nitro-L-arginine methyl esther (250 mu g.mL(-1)) from 145 +/- 199 to 38 +/- 5 mu m), but this was not statistically significant (n = 6, P = 0.18). Intra-arterial infusions of D-glucose (20-40 mmol.L-1.min(-1)), during graded constrictions, caused statistically significant increases in blood flow (n = 11, P = 0.0013). Vasodilatation was confirmed by measurements of the ratio of immediate pressure steps to flow steps (partial derivative P/partial derivative F) during the graded obstruction experiments, showing a decrease in instantaneous vascular resistance from a control of 0.62 +/- 0.30 to 0.33 +/- 0.34 mm Hg.mL(-1).min(-1) (n = 7, P = 0.016). Autoregulation was assessed from the slopes of the plots of steady-state flow versus pressure. There were significant increases in the slope from 2.32 +/- 1.03 to 5.88 +/- 5.60 mL.min(-1).(mmHg)(-1) (n = 7, P = 0.0078), indicating significant impairment of autoregulation. In conclusion, luminal hyperglycaemia relaxes both arterial and resistance vessel smooth muscle.