Aims-Ophthalmodynamometry has been used extensively since the last century; however, controversy surrounds what it actually measures. This study was set up to determine the relation between ophthalmodynamometric (ODP) and systemic blood pressures.Methods-Aortic pressure was continuously monitored and altered by phlebotomy in six anaesthetised dogs, while ophthalmodynamometry was performed, by directly altering intraocular pressure. Maxillary artery pressure was monitored in two animals. All pressure transducers were zeroed at eye level.Results-Mean ODP was 96.6% (1.6%) (95% confidence interval, n = 49) of aortic pressure. Mean maxillary artery pressure was 95.7% (5.5%) (95% CI, n = 16) of aortic pressure. ODP was 1.9 (0.6) mm Hg (95% CI, n = 33) higher than maxillary artery pressures.Conclusion-ODP was only slightly below aortic pressure and not significantly different hom maxillary artery pressure, the analogue of the internal carotid artery in humans. These results also suggest a retinal artery collapse pressure of at least 1.9 mm Hg.