SUMMARY1. Nasal vascular and airflow resistances have been measured in dogs, simultaneously on both sides separately.2. Vascular resistance was measured either by constant flow perfusion of the terminal branch of the maxillary artery (which supplies, via the sphenopalatine artery, the nasal septum, most of the turbinates and the nasal sinuses) or by measuring blood flow through this artery, maintained by the dog's own blood pressure.3. Airflowresistancewasassessedbyinsertingballoon-tippedendotrachealcatheters into the back of each nasal cavity via the nasopharynx, and measuring transnasal pressure at constant airflow through each side of the nose simultaneously.4. Preliminary experiments indicated that there was 5-10 % collateral anastomosis between the two sides.5. Close-arterial injection of drugs showed different patterns of response. 6. Adrenaline, phenylephrine, chlorpheniramine and low doses of prostaglandin F2a increased vascular resistance and lowered airway resistance.7. Salbutamol, methacholine and histamine lowered vascular resistance and increased airway resistance.8. Dobutamine decreased airway resistance with a small increase in vascular resistance.9. Prostaglandins E1, E2 and F2a (high dose) decreased both vascular and airway resistances.10. Substance P, eledoisin-related peptide and vasoactive intestinal polypeptide lowered vascular resistance with little change in airway resistance.11. The results are interpreted in terms of possible drug actions on precapillary resistance vessels, sinusoids and venules, and arteriovenous anastomoses. It is concluded that nasal airway resistance cannot be correlated with vascular resistance or blood flow, since the latter has a complex and ill-defined relationship with nasal vascular blood volume. M. A. LUNG AND OTHERS