1. Vasoactivity of human A- and B-calcitonin-gene-related peptides was studied in normal subjects, using reflectance plethysmography to measure relative changes in blood flow produced by superficial subcutaneous injections of the peptides. 2. Injection of 10(-11) mol of either peptide caused an immediate 200% increase in local blood flow and prolonged hyperaemia lasting up to 3 h. The hyperaemic response to 10(-13) mol of each peptide was significantly (P less than 0.01) smaller and shorter in duration than that elicited by 10(-11) mol of the same peptide, and 10(-15) mol of both peptides produced no hyperaemia other than that attributable to needle insertion alone. 3. At all three dosages examined, there were no significant differences between A- and B-calcitonin-gene-related peptides in magnitude or time course of the hyperaemic response. 4. Human A- and B-calcitonin-gene-related peptides are therefore potent vasodilators in man, causing comparable dose-related vasodilatation in the superficial tissues.