The effect of a purified bovine thymus gland extract (Dr. Kurt Mulli, GmbH, Hamburg, West Germany) was studied in 12-week old male Wistar rats on the following: thyroid weights and morphology, T3-T4 serum levels, thyroid lactic dehydrogenase, ATP-ase, acid phosphatase, and non-specific esterase activities. Thymus extract was administered intramuscularly daily for 21 days at doses of 0.5, 1.0 and 2.0 ml/kg. Measurements were made on day 3, 7, 14 and 21 of treatment. Thyroid histology and enzyme activity were studied only on 21-day specimens. Thymus extract significantly decreased average thyroid gland weights in a dose-dependent manner irrespective of treatment duration. T3 serum levels were consistently lower in thymus-treated rats irrespective of treatment dose or duration. Changes from control levels were not statistically significant due to large standard deviations. T4 serum levels were significantly lower than control levels in rats treated with thymus extract for 14 and 21 days. Histology of thyroids from 21-day treated animals revealed a marked reduction in both thyroid follicle size and colloid content with an increase in connective tissue, resorption vacuoles and hyperemia. Histochemical study of thyroid enzyme activity showed lactic dehydrogenase increase in follicle epithelial cells, acid phosphatase increase in follicle epithelial cells and decrease in interstitium, ATP-ase increase in granular storage area and non-specific esterase increase in follicle epithelial cell. The data suggest the presence of an unidentified specific anti-thyroid factor(s) in the thymus gland extract.