Male Swiss albino mico were exposed to whole-body irradiation by fast neutrons of 14 MeV average energy. Two single doses of 7 or 14 rem were used, corresponding to a fluence of 1.27×108and 2.54×108n/cm2, respectively. Two enzymes were assessed in testicular tissue, acid phosphatase activity to measure the changes in lysosomal function and succinic dehydrogenase activity to test mitochondrial functions and energy production. Lysosomal affection was revealed by statistically significant increase of ACP activity in all cell types of testicular tissue with either of the two doses used. Although SDH was characterised by relatively low activity in most of the testicular tissues, decrease in enzyme activity was clear. Complete absence of activity was sometimes noted. The magnitude of response was dose dependent and there was a tendency to return to pre-irradiation levels of both enzymes with time.