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In our experiments, carried out hitherto, concerning the effect of incorporated and radioactive substances, weight behaviour and food uptake have proved to be a sensitive test. With regard to these experiments and the half-life of the radionuclides used, it is reported about trial series in Wistar rats. These rats were applied, with Co-60 gamma irradiation, different whole-body doses protracted over 48 hours. A total of 32 groups of experimental animals (20 animals each) was exposed to irradiation doses of lethal, medium lethal, and sublethal ranges, control and pseudo-irradiation series included. The experiments were carried out under observance of constant irradiation and attitude conditions, night and day changes, as conditioned by the season, included. Even in the inferior sublethal range (12 to 24 R), a significant trend of decreased food uptake is registered. This trend remains for a short period after the end of irradiation, but then it returns to normal conditions. Furthermore, a new decrease with subsequent increase seems to become evident-about ten days after termination of the radiotherapy (especially after several hundred R); report about these items will be made later on.

Extracorporeal measurement of incorporated radioactive organic compounds needs labelling with gamma emitters. The elements of organic compounds have only very short-lived gamma-emitting isotopes. (Organic compounds, therefore, up to now were labelled by more or less stable chemical coupling with gamma-emitting isotopes of the halogens or of certain metals as, for instance, chromium-51). The use of the C-11 isotope with a half-life period of about 20 min, emitting gamma radiation due to the annihilation radiation of its positrons, is very problematical, because these very short-lived nuclides can be used only at the place of their production by nuclear engineering. But especially these nuclides are very interesting just because of their short half-life period, the radiation load being lessened thereby and, in addition, the organic compound remaining chemically unchanged. Utilizing the (gamma, n) reaction, we have succeeded in labelling organic molecules in solid state directly within the electron beam due to the retarding of particles from a linear electron accelerator, and the absolute as well as the specific yield was satisfying. Till now, we have tested almost exceptionally amino-acids; during experimentation with rats, an example of the practicable utilization in nuclear medicine was set by means of the amino-acid methionine. This was confirmed by results of comparative studies using C-14 and Se-75 labelled methionine (Se-methionine respectively).

The influence of the metabolic status of mitochondria upon their radiation response was studied. It turned out that the reduction of pyridine nucleotides by addition of beta-hydroxybutyrate (BOH) and the decrease through respiration were distinctly impaired in mitochondria with a substrate deficit by an irradiation with 2.7 krd already. The evolution of the lesion depends on the incubation conditions following irradiation, a presence of BOH having protective effects. The general redox level seems to be essential for radiation response of mitochondria. An exit of NAD from mitochondria has proved to be the main cause for inhibition of the respiratory BOH-consumption.