In order to evaluate the toxic effects of Sodium Cyanate (NaOCN), it was orally administered to growing mice at sea level (SL-CN) and to mice chronically exposed to intermittent hypobaric hypoxia (IHH-CN). The effects on body weight, in-vivo O2 consumption (VO2) and the respiratory function of liver mitochondria were evaluated. At sea level, the animals on cyanate lost weight in contrast with the controls that gained weight. When exposed to IHH, the controls lost weight and the animals on cyanate regained weight. After 2 months observation the weights of the IHH-CN and IHH-C were similar. The VO2 after one month of treatment was similar in the SL-C and in the SL-CN but it was lower in the IHH-CN when compared with IHH-C. The substrate-stimulated respiration of isolated liver mitochondria (ST4) was not affected by NaOCN, but the ADP-stimulated respiration (ST3) was reduced. The ratio ST3/ST4 (RCR) was also lower. These changes were present in both SL and in IHH and were much larger after three months of treatment. The toxic effects of chronic administration of NaOCN are discussed.