Studies were carried out on 7 sheep aged 2-4 years about 40 kg in body weight successively in summer and autumn. The animals were fed in 6 feeding groups. The samples of arterial and venous blood and control air also were taken 4 hrs after morning feeding. During feeding sheep were given in a short intraruminal 1-hr infusions unsaturated fatty acids: linoleic and cis-oleic. The methane levels in arterial and venous blood were both higher in summer than in autumn. The contents of CH4 in each of the diets were higher in venous blood than in arterial blood. The highest inhibition of methane production was observed in blood of experimental sheep which were fed on a bulky feed.