Analysis of the gas contained in the cavities of plants has attracted the attention of many investigators. Such analyses have usually been in relation to photosynthetic and respiratory exchanges, and therefore have considered only oxygen and carbon dioxide, residual gas being termed "nitrogen. ' ' The presence of 1 to 12 per cent, of carbon monoxide was reported in the marine brown alga Nereocystis luetkeana by Langdon (5) and confirmed by Langdon and Gailey (6). This seems to be the first and only recorded instance of carbon monoxide being produced by a plant, which therefore merits further study, to which this paper is a contribution. It is of interest to note that the presence of carbon monoxide was claimed in the blood of mammals, including man, in 1898 by Saint-Martin (20) and by Desgrez and Nicloux (1), and in a series of papers by Nicloux (11, 12, 13, 14, 15) who considered the carbon monoxide to be a normal product of metabolism. Several investigators have reported its presence in small quantities in human blood, including McIntosh (10), Rathery et al. (16, 17), Gettler and Mattice (4), Loeper et al. (7, 8, 9), Roffo (19) and Schmidt (21). Nereocystis luetkeana has been described by Langdon but its main characteristics may be recapitulated. It is an annual found only in the sublittoral zone along the west coast of North America. The plant consists of a branched holdfast adhering to the rocks, a long, slender stipe extending from the holdfast to the water surface, and long, ribbon-like fronds attached to the free end of the stipe. The cylindrical stipe is the portion of particular interest. Its growth is very rapid (Sheldon) which results in the attainment of an extreme length of 20 to 25 meters in a single season. Its circumference gradually increases with distance from the holdfast, the stipe terminating in a bulb-shaped end 5 to 10 cm. in diameter. With this increase in circumference a longitudinal cavity develops in the center, leaving a surrounding wall 0.5 to 1.5 cm. in thickness. This hollow portion, or pneumatocyst, is buoyed up by the gas within it. The pressure and composition of this gas are the subject of the present study. Frye (3) found in the examination of a thousand specimens that the average pressure of the gas in the pneumatocyst was 77 mm. of mercury less than atmospheric pressure, the pressure ranging from 316 mm. less to 124 mm. greater. He showed that the pressure in a specimen was highest at 3-4 p.m. and lowest at 7 a.m., averaging 16 mm. difference.