Vasoconstriction during and after microsurgery may cause hypoperfusion and result in flap necrosis. This study investigated the vascular effects of two known vasodilators: lidocaine and the naturally-occurring substance, calcitonin gene-related peptide (CGRP). Experiments were performed in vitro on 47 human radial arterial rings from 18 patients undergoing reconstruction after head and neck tumor surgery with free radial forearm flaps. Lidocaine produced a biphasic dose-response curve, with contraction at low concentrations (1.5 x 10(-5) to 1.5 x 10(-3) M) and relaxation at higher concentrations (4.5 x 10(-3) to 1.5 x 10(-2) M). It caused significantly stronger contractions in arteries with mechanically removed endothelium, compared with arteries with intact endothelium. Prolonged rebound contraction occurred when the lidocaine concentration was decreased. Lidocaine also potentiated noradrenaline-induced contraction. CGRP produced only dose-dependent relaxation of constricted radial arteries at low concentrations (3 x 10(-10) to 10(-7) M). The contractile effects of lidocaine should be considered when a vasodilative substance is required.