i and 2) Brandt (1899) first suggested that phytoplankton organisms must, by removing from the illuminated surface layers of the sea the nutrients required for their further growth, place a limit on their continued multiplication, and he and Raben for long worked on the problem (see Brandt, 1927, for review of earlier work). The thesis has been proved true beyond all doubt by work which has followed improvements in analytical technique by Atkins and Harvey. In many waters reserves of nitrate are formed during the winter months and are used up, often completely, with the coming of longer days. Interest is now centred on the mechanism by which the nitrate reserve is built up and depleted and on the role played by other compounds of nitrogen. The time has come to review the nitrogen problem as a whole and to assess the importance of many possible reactions in which nitrogen may be involved. The plan of this paper is based on Fig. 1, in which associated reactions are N204 -»-NMejO--NHMe, + CHjO Fig. 1. The numbers indicate the reaction group in which the reaction is considered in the text.