Brandt (1927) has summarised and reviewed our knowledge of the nitrogen compounds existing in sea-water, including nitrites. In general, estimations of nitrite in sea-water have been made colorimetrically, using the Griess-Ilosvay reagent, which has also been used in the present work. Usually the amount of nitrite found, recorded as milligrams of nitrogen per cubic metre, was small. Raben's results have been grouped by Brandt as follows, 0 mg., 48 cases; under 1 mg., 53 cases; 1–4 mg., 21 cases; 5–9 mg., 6 cases. Orr (1926) in the Clyde Area, found from under 1 mg. to about 10 mg. in June 1925, the deeper water having more than the upper layers. Harvey (1926) states that, “in a number of. water samples from the English Channel, examined either on board ship or immediately after landing, no definitely detectable traces of nitrites could be found either by the Griess-Ilosvay reagent or with the strychnine reagent without sulphuric acid, except in the case of inshore water subject to pollution. After storage-an appreciable quantity of nitrite has been detected in several instances. In water from 830 metres in the Bay of Biscay, preserved with mercuric chloride when collected, and in water at various depths up to 3000 metres, similarly preserved, collected by the R.R.S. Discovery some 200 miles west of Portugal, no nitrite could be definitely detected.”