1. The determination of organic nitrogen by the KJELDAHL method is reviewed up to 1950. 2. A method is described in which samples of biological material containing from 0.14 to 0.70 mg. N were employed. The accuracy of the obtained results was evaluated and their standard deviation was found to be 0.0007 mg. Research is beeing carried out on kjeldahlization curves of different materials which is the object of a further paper.