Acetylcholine (ACh) induces nasal congestion at low doses but decongestion at high doses. The current study investigated the vascular mechanisms underlying this biphasic nasal airway response in dogs. Collecting and outflow veins from anterior and posterior nasal venous systems and the septal mucosa (containing sinusoidal venous plexuses) were isolated. The in vitro isometric tension of the vascular segments was monitored to reflect vascular reactivity. Immunohistochemical localisation of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase and endothelial nitric oxide synthase (eNOS) was performed. ACh did not affect the venous plexuses but contracted the anterior collecting vein and the outflow veins of both systems in a concentration-dependent manner; the responses were unaffected by nitro-L-arginine-methyl-ester (L-NAME). ACh relaxed posterior collecting veins at low concentrations but contracted them at higher concentrations; L-NAME enhanced the contractions but inhibited the relaxations, with the inhibition reversed by L-arginine. NADPH-diaphorase and eNOS were located predominantly in the posterior collecting veins. The fact that acetylcholine at low concentrations relaxes posterior collecting veins but contracts other collecting and outflow veins implies that the agonist in vivo may induce nasal congestion by increasing posterior blood volume. At higher concentrations, acetylcholine contracts posterior collecting veins as well, implying diminished blood volume in both venous systems, and consequently nasal decongestion. The induced contraction in posterior collecting veins is nitric oxide-independent, while the induced relaxation is nitric oxide-dependent.