Abstract Four rumen fistulated wethers maintained on grass hay were used in a water bath test designed in a 4 × 4 Latin square. Nitrate (1.3 g NaNO 3 kg −0.75 body weight) was administered into the rumen without or with l-cysteine (0.21 g sulphur equivalent kg −0.75 body weight) or sulphide (0.105 sulphur equivalent kg −0.75 body weight) as a single dose 30 min after the morning feeding. Physiological saline was administered as control treatment. The water bath test was performed 2.75 h after administration of treatments by immersion of the animals in water at 38 °C and 18 °C. Respiratory gas exchange and metabolic rate were monitored by an open-circuit respiratory system during the water bath tests. Administration of nitrate without sulphur compounds induced an increase in methaemoglobin to 21% of total haemoglobin and a decrease in respiratory gas exchange and metabolic rate. Consequently, during exposure to 18 °C the rate of decrease in rectal temperature of the nitrate treated animal decreased at a two-fold rate of those on the saline treatment. Both l-cysteine and sulphide effectively counteracted the metabolic, Iterations induced by nitrate.