1. Our aim was to determine whether the vasodilating substance nitric oxide (NO) contributes to the rise in forearm blood flow observed during mental stress in humans. We also determined whether the NO might be released as a result of cholinergic stimulation of the vascular endothelium. 2. Blood flow was measured in both forearms using plethysmography during several 3-5 min bouts of a colour word test. In one forearm the nitric oxide synthase blocker NG-monomethyl-L-arginine (L-NMMA) and other drugs were infused via a brachial artery catheter. The contralateral forearm served as a control. 3. When L-NMMA was given prior to mental stress it blunted the rise in blood flow in the treated forearm almost completely. The normal blood flow response returned during a second bout of stress conducted after a wash-out period. During a third bout of mental stress, administration of more L-NMMA again blunted the blood flow responses to mental stress. 4. When atropine was given prior to mental stress, the increases in blood flow were reduced in the treated forearm. Subsequent administration of both atropine and L-NMMA caused a somewhat greater reduction in the blood flow responses than those observed with atropine alone. 5. These data demonstrate that NO plays a role in forearm vasodilatation during mental stress in humans. It is likely that most of the NO is released by cholinergic stimulation of the vascular endothelium.