The aim of the present study was to evaluate the use of biochemical parameters measured in the venous blood in conscious and isoflurane anesthetised sheet for monitoring the acid-base status and the blood gas analysis as an alternative to arterial samples. Arterial and venous blood samples were collected ischronously by punctures of the femoral artery and the jugular vein respectively in 8 adult sheet before and under isoflurane anaesthesia and the pCO(2), tCO(2), pO(2), O(2sat), pH, HCO(3)(-), BE (base excess of extracellular fluid) as well as the hematocrit, the haemoglobin and electrolyte (Na(+), K(+) and Ca(2+)) concentration were measured on the 2 blood types. Venous and arterial values did not significantly differ except for pO(2), pCO(2). BE and hematocrit: whereas arterial pCO(2) was significantly depressed in conscious sheet compared to the venous value, the arterial values of the hematocrit. BE and pO(2) were significantly higher, particularly in anaesthetised animals. Nevertheless, according to the Bland-Altman procedure, sufficient agreements were evidenced for all the parameters investigated except for the pO2 values. For this reason and because the O(2sat), and pO(2) values evaluate in the same way the tissue oxygen availability, it is concluded that venous blood gas values instead of arterial data can be used for the determination of the acid-base and oxygenation status in conscious and isoflurane anaesthetised sheep.