1. A method for micro respiratory studies is described with a sensitivity down to 0.2 millionth of a cubic millimeter and a stability of 10-0 millionths of a cubic millimeter an hour. It is basically a constant volume method, where a minute reference diver is introduced into the chamber together with the cell. A bubble is generated on the diver until it just floats. The change in flotation pressure multiplied by a factor of known constants gives the oxygen consumption. The CO2 is effectively held in solution by the high liquid-to-gas ratio. The system has been used to study the respiration of single protozoans and to follow in detail the oxygen consumption during cleavage of single eggs of several species, which will be reported elsewhere. 2. A new micro method is described for direct determinations of gas tensions in liquids using the reference diver.