Intra-ocular pressure (IOP) variations during extracorporeal circulation (ECC) and some influencing factors were studied prospectively in 38 patients undergoing cardiovascular surgery. IOPs were measured pre-operatively, during ECC (after 5 min, 10 min, 25 min and every 15min thereafter), and on the 1st, 2nd and 3rd postoperative days. IOP, haematocrit, arterial perfusion pressure, partial O<sub>2</sub> pressure, partial CO<sub>2</sub> pressure, pH and blood bicarbonate levels were measured simultaneously. After the beginning of ECC, arterial perfusion pressure and haematocrit values decreased suddenly, while IOP raised immediately. Sudden increase of blood volume in the beginning of ECC can be the probable cause of IOP elevation. Afterwards, IOP levels decreased to pre-ECC levels. We think that the drop is due to the effect of mannitol in the priming solution and hyposecretion of aqueous humour due to insufficient nutrition-oxygenation of corpus ciliaris after haemodynamic stabilization.