The aim of the present study was to determine the acid-base balance in the blood of the Lacaune sheep during different lactation stages. Thirty lactating Lacaune sheep were involved in the research, and were monitored in the early (day 60), medium (day 120), and late lactation stages (day 180). The following parameters were determined in the blood plasma: pH, partial pressure of carbon dioxide – pCO2, partial pressure of oxygen – pO2, total pressure of carbon dioxide – tCO2, oxygen saturation – sO2, actual base excess Cbase – B, standard base excess Cbase – Ecf and electrolytes (Na+, K+, Ca2+, Cl- and HCO3- – bicarbonate), while total proteins – TP and albumin ALB were determined in the serum. Also, strong ion difference - SID, z value, anion gap - AG, weak anions, and cations - ATOTtp and ATOTalb, strong ion gap - SIG, unmeasured anions – XA and globulin concentrations were calculated. A significant effect of the lactation stage on most of the acid-base parameters in the blood of the Lacaune sheep was determined, except for pCO2, Cl-, z values and the SID. A significant increase of pO2, sO2, ATOTtp, ATOTalb, AG, SIGalb, SIGtp, XA, Na+, total proteins and albumins, as well as the decreased concentrations of K+ and Ca2+, were determined in the blood of the Lacaune sheep during lactation. During the medium stage of lactation, significant changes were determined in most of the acid-base parameters in the blood of the Lacaune sheep when compared to the other stages. A lactation stage may significantly affect the acid-base balance parameters in the blood of the Lacaune sheep, although the calculations of SIG, base excess (BE), and unmeasured anion (XA) values should also be included for the obtainment of a more comprehensive picture of the acid-base balance.