Histamine antagonists in current clinical use have not yet been administered in intr-arterial studies. We intend to use chlorphenamine (H1 antagonist) and ranitidine (H2 antagonist). Intravenous (IV) administration of 10mg chlorphenamine results in a plasma concentration of ~14 mcg/l. IA administration of 1 mcg/min would achieve a similar forearm concentration. Assuming NAC causes vasodilatation with an increase in forearm blood flow, the investigators propose to administer 5 mcg/min to ensure maximal H1 blockade. Similarly, IV administration of 50mg ranitidine results in a plasma concentration of ~150 mcg/l. IA administration of 7.5 mcg/min would be expected to achieve a similar forearm concentration. In the presence of increased forearm blood flow, the investigators propose to administer 37.5 mcg/min.