Forty-two mature albino rats were subjected to the ligation of selected lymphatic vessels close to, and at a distance from, the testis. At periods from 3 days to 1 year both testes were studied at their equatorial level in order to determine histological changes. The operated testes showed a mild degree of damage 3 days after operation and moderate damage at 7 days, while damage of a severe degree increased to a maximum over a postoperative period of 90 days. One-hundred-and-eighty days after operation a few tubules presented the typical appearance of spermatogenic destruction. After 1 year recovery is virtually complete. Changes in the contralateral testis indicate an immune response to the damage induced on the ipsilateral side by the experimental interruption of selected lymph vessels, where interstitial oedema was confined sectorially to the medial half of the testis. In the contralateral testis, oedema was predictably diffuse throughout; the distribution of tubular deformities and the degree of cellular damage were less well marked and 1 year after operation the testis had almost completely recovered. Tubular deformities and cellular changes in the germinal epithelium are primarily dependent on interference with the lymph flow and are not secondary to ischaemia.