Tetracycline was administered orally to Shinshilla male rabbits (female, 2.0-2.5 kg) twice a day, the daily dose being 20 mg. The thymus was examined in 1 and 2 weeks by the methods of histo- and cytometry. The reliability of the differences and correlation were estimated by the non-parametric methods. Out of 15 parameters studied 8 showed reliable differences and 5 were statistically connected. The following structural shifts were found: (1) increased numbers of activated and middle thymocytes evident of intraorgan cell activation, (2) higher mitotic index and lower index of the cell degradation resulting in the cortex hypertrophia by the end of the week, (3) increased cell density in the medullar substance which is the morphological criterion of intraorgan migration of the cells from the cortex into the medullar substance. Thus, the morphometric analysis of the thymus proved to be much more informative than the usual morphological analysis. The observed structural change in the thymus is considered to be a result of the antigenic effect of tetracycline on the host.