When a moderately strong solution of sodium nitrite is added to blood a change of colour almost immediately occurs, mainly owing to the formation of methæmoglobin. The actual products of the reaction are not definitely known, and are not of importance to the present paper. The rapidity of the reaction is affected by the nature and concentration of the blood solution on the one hand, and by the concentration of the sodium nitrite solution on the other. Probably other factors, such as temperature, which has not been investigated, are of importance. The experiments referred to in this paper were made at room temperatures.