In view of the importance of sulfhydryl groups in producing a chemical protection against ionizing radiation, an attempt was made to evaluate the changes of intrinsic non-protein sulfhydryl (NP-SH) and non-Protein disulfide(NP-SS) of the lung and liver tissues of rabbits following the whole body X-irradiation with 900 r either in single or fractionated (300 r X 3) dose. NP-SH was measured by Ellman s method, and NP-SS was measured by the electrolytic reduction method described by Dohan. Experiment was performed at 1,3,5,24 and 48 hours post¡¤irradiation, and the results were compared with the control. The results thus obtained are summarized as follows; 1) Intrinsic levels of NP-SH and NP-SS of normal rabbits were 0.77±0.10 and 0.61±0.07 μmol/gm wet weight in liver, and 0.28±0.03 and 0.54±0.03 in lung tissues respectively. 2) NP-SH of liver after single X-irradiation showed no significant change in general, but at 48 hours post¡¤irradiation, it was elevated comparing with the normal value. 3) Levels of NP-SS in liver was decreased than the normal value in the irradiated groups, and the lowest level was observed at 3 hours after single X-irradiation and at 5 hours after fractionated irradiation. 4) In lung tissues, levels of NP-SS showed no significant change from the control at earlier experimental hours, but a great decrease was observed at later Part of the experiment.