We studied effects of mental stress on whole-blood viscosity (WBV) and blood pressure (BP), and relations between WBV and autonomic nervous system activity and insulin sensitivity. We measured WBV (rotational rheometer), plasma noradrenaline (NA), finger BP, heart rate variability (HRV) and baroreflex sensitivity (BRS; transfer technique) during hyperinsulinaemic glucose clamp and mental arithmetic stress test (MST) in 20 men with high ( > or =140/90 mmHg) and 21 men with normal (< or =115/75 mmHg) screening BP, and 10 women regardless of screening BP (all normotensive). WBV and NA increased during the MST, while HRV and BRS decreased. During the MST, WBV (all shear rates) and the response ((delta)WBV) (low shear) were higher in men with high compared to normal screening BP (p<0.05). In men, WBV correlated positively with NA and negatively with HRV, BRS and insulin sensitivity. The diastolic BP response ((delta)DBP) was independently explained by high-shear (delta)WBV (p<0.05) and (delta)NA (p<0.0001), and (delta)WBV independently by (delta)DBP (p<0.05). WBV is related to increased sympathetic activity, impaired vagal cardiac control and low insulin sensitivity in young adults. The haemorheological effect of mental stress is increased in young men with high screening BP and may be mediated by the acute increase in BP.