AbstractHaemoglobin, methaemoglobin, and NADH‐methaemoglobin reductase were investigated in sheep blood after intraruminal administration of DNBP and sodium nitrite at doses of 45 and 75 mg/kg b. wt., respectively. The sheep suffered from methaemoglobinemia at maximum levels of 5.5–7.5 g MetHb/100 ml blood after about 3 hours. The nitrite‐induced methaemoglobinemias were normalized during the first 12 hours, while the DNBP‐induced methaemoglobinemias lasted for 2–3 days or more. DNBP also caused serious haemolyses in the sheep after 1–2 days, and two of these animals died. The normal level of methaemoglobin reductase in 10 sheep was 3.3 ± 0.31 units. Nitrite had only a moderate and temporary effect on the reductase enzyme activity, while DNBP had a progressive and almost complete inhibitory effect o