1 Various doses of mediators were tested on tracheal vascular resistance in dogs anaesthetized with pentobarbitone. Tracheal vascular resistance was determined by perfusing the cranial tracheal arteries at constant flows and measuring inflow pressures. 2 All drugs produced dose-related changes in vascular resistance. 3 The peptides bradykinin, substance P and vasoactive intestinal peptide (VIP) each had similar vasodilator potencies and were much more powerful than histamine, methacholine and salbutamol. 4 Platelet activating factor (Paf) was a weaker dilator than the peptides. Prostaglandins D2, E1 and F2 alpha had wide dilator potency ranges, PGE1 being very effective even at low concentrations. 5 Phenylephrine, an alpha-adrenoceptor agonist, was the only drug tested that always increased vascular resistance. 6 All the drugs studied also had effects on the contralateral tracheal vascular resistance.