There was no visible thymus in ageing rats of 18 months, and 7 days after orchidectomy there was still no evidence of a thymus. By 30 days after the operation, however, there was a well-defined and well developed bilobular thymus overlying the heart, although it was smaller than those observed in 10-week-old rats. Histologically, the tissue appeared normal, was well vascularized, filled with lymphocytes and several mitotic figures were also seen. When compared with sham-operated animals, blood from these animals had a significantly higher lymphocyte count. These results have important implications for the possible enhancement of the immune system with associated improvement of health during ageing.