The catalytic activity of La1-xSrxMO3 perovskites (M = Fe or Co) was investigated for the reduction of nitrogen monoxide. The catalytic activity for the NO + CO and the NO + H2 reaction was investigated in a fixed bed reactor in the temperature interval 873-1223 K. The perovskites showed high activity for the reduction of nitrogen monoxide in the absence of oxygen even at 873 K. Hydrogen was less active than carbon monoxide as a reducing agent and the cobalt perovskites showed slightly higher activity than the iron perovskites in the lower temperature range. The reduction of nitrogen monoxide was concluded to proceed through the reduction of the surface by the reducing agent followed by adsorption and decomposition of nitrogen monoxide on the surface. The activated (reducing) perovskites were investigated by X-ray diffractometry. The cobalt perovskites were reduced to a compound of the K2NiF4 type, La2CoO4, while the iron perovskites retained their perovskite structure upon reduction. The perovskite structure of the cobalt perovskite was restored upon reoxidation of the La2CoO4 compound. The introduction of excess oxygen into the gas mixture decreased the nitrogen monoxide reduction level to zero. Nitrogen monoxide (1000 ppm) was completely reduced in a stoichiometriC CO + 02 MiXture (2% CO and 1% O2) over La0.8Sr0.2CoO3 at 1173 K. LaCoO3 showed high thermal stability in a helium atmosphere at 300-1223 K, while the perovskite was reduced to La2O3 and cobalt metal in a 10% CO + He atmosphere.