The rate of ammonia synthesis was measured for ruthenium and iron catalysts under high (100 bar) and under atmospheric pressures. The effect of ammonia concentration in the gas phase on reaction rate was measured at 400, 430, and 470 degrees C for p = 100 bar at 370 and 400 degrees C for p = 1 bar. It was found that ruthenium is less sensitive than iron to the increase of ammonia pressure in the gas phase. It is also much less sensitive to changes in the total pressure (for p(H2):p(N2) = const.). At relatively high conversion degrees the ruthenium catalyst used was several times more active than the fused iron catalyst at 100 bar and 25-40 times more active under atmospheric pressure.