Microdiffusion methods for determinations of minute amounts of ammonia, urea, carbon dioxide, the halogens etc., are now established in many laboratories. A general account of apparatus and several applications have been given elsewhere [Conway, 1939]. Recent extensions include submicro-determinations of total N by Needham & Boell [1939], as well as a further very accurate submicro-total N method by Tompkins & Kirk [1942], also amideand nitrate-N etc. by Borsook & IDubnoff [1939], quantitative acetone determination by Werch [1940] and an accurate acetone method using bisulphite absorption by Winnick [1941; 1942], who has also extended the technique to lactic acid and threonine estimations. As shown elsewhere micro-NH3 determinations by the microdiffusion technique can be brought to any desirable level of accuracy. Recoveries are better than with aeration methods, the apparatus required,is simpler and large numbers of determinations can be carried out in one series. Using the standard unit and 0-2 ml. of the NH3-containing fluid the full absorption time at room temperature is 45 min., but with rocking this can be reduced to 10 min. Such times arise out of the principles discussed and the experimental results described. (Borsook & Dubnoff [1939] have erred in claiming an absorption time of 90 min. as an advance on the microdiffusion technique.) The present study is directed especially to the removal of certain objections to the clinical use of the microdiffusion method for urea, but has also a more general application for concentrations of NH3 beyond a certain level. The blood urea method as originally described [Conway, 1933] had the disadvantage for clinical use, as commented on by Lee & Widdowson [1937], that the upper limit was 100 mg. urea/100 ml. Occasional blood ureas exceed this figure, though as pointed out