The underlying pathophysiological triggers for equine acute laminitis are unknown, although digital vasoconstriction, ischaemia, hypoxia and reperfusion injury may be involved. The contractile responses of isolated equine digital arteries (EDAs), harvested from the hindlimbs of normal horses postmortem at an abattoir, were studied acutely (up to 3 h) under hyperoxic (95% oxygen, 5% CO2) and hypoxic (95% nitrogen, 5% CO2) conditions in organ baths. Phenylephrine (PHE; 10-6 m), 5-hydroxytryptamine (5-HT; 10-7 m) and high potassium (K+; 118 mm) caused contraction in EDAs which was significantly (P < 0.0001) enhanced under hypoxic conditions. In contrast, contraction stimulated by 9,11-dideoxy-9α,11α-epoxymethanoprostaglandin F2α (U44069; 3 × 10-8 m) was not significantly enhanced by hypoxia (P = 0.75). Hypoxia-enhanced contraction in response to K+ was greater (P < 0.03) in vessels with a functional endothelium than in vessels in which the endothelium was removed by rubbing. Fasudil (10-6 to 10-5 m), a Rho kinase inhibitor, and apocynin (10-3 to 3 × 10-3 m), an NADPH oxidase inhibitor, significantly (P ≤ 0.05) inhibited hypoxia-enhanced contraction in response to PHE and 5-HT. In conclusion, hypoxia-enhanced contraction occurred in EDAs. This appears to be partially mediated by reactive oxygen species produced by NAPDH oxidase, which activate Rho kinase to increase calcium sensitisation and enhance smooth muscle contraction.