THE soil percolation technique1 was introduced for the study of soil metabolism. In this technique small volumes of a water solution of metabolites are taken regularly and frequently from a reservoir, allowed to percolate through a column of aerated soil, and returned to the reservoir. The course of soil metabolism is then followed by analyses of the reservoir fluid. The original apparatus2 has since been modified3 and adapted to the measurement of the carbon dioxide outputs of percolated soils1. The apparatus to be described permits the measurement of the oxygen uptakes of percolated soils or indeed of any metabolizing system capable of being supported on a solid porous medium.