· Background: A study was performed to evaluate the effect of isoxic hypercapnia on ocular haemodynamics using colour Doppler imaging (CDI), scanning laser Doppler flowmetry (SLDF) and ocular blood flow (OBF) tonography. · Methods: Measurements were taken for one eye of each of 14 healthy subjects (mean age 27±6 years) during breathing of room air and then during isoxic hypercapnia (breathing CO₂ and room air). Using CDI, blood flow velocities and resistance indices were determined for the ophthalmic artery (OA), central retinal artery (CRA) and short posterior ciliary arteries (SPCAs). Using SLDF, a 10×10 pixel frame was used to measure blood flow, volume and velocity in each quadrant of the peripapillary retina. Pulsatile ocular blood flow (POBF) was measured using the OBF tonograph. · Results: Using CDI, peak systolic and end diastolic velocities increased and resistance index decreased significantly in the SPCAs during hypercapnia. Using SLDF, blood flow, volume and velocity increased significantly during hypercapnia in the superior temporal quadrant of the peripapillary retina. No significant difference was observed between baseline and hypercapnia for POBF. · Conclusions: Isoxic hypercapnia resulted in an increase in peripapillary retinal and SPCA blood flow parameters as determined by SLDF and CDI respectively. This implies the presence of autoregulatory activity in these vasculatures. These findings may be of significance in the pathogenesis of ocular disease such as glaucoma where autoregulation is thought to be compromised.