[en] Preliminary results are reported for a photon activation method for measuring in vivo the total body content of the elements carbon, nitrogen, and oxygen. In vitro experiments indicate that the method has a reproducibility of 2% (C), 7% (N), and 2% (O), for an estimated peak radiation dose of approximately 2 cGy in a human subject using a 4000 cm³ NaI(Tl) detection volume. The proposed method has been tested on rats against a chemical digestion technique with excellent results for carbon but poor results for nitrogen. It is felt, however, that the latter is not due to problems with the proposed method. Technology transfer is discussed and it is felt that bremsstrahlung x-ray sources of 25 MVp or higher would be suitable to apply this method to human subjects at a clinical level. (author)