We used immunohistochemistry with monoclonal antibodies (TRPM1, TRPM2) and histochemistry (acid phosphatase (AcP)) to investigate the effects of cyclosporin A (CsA) on macrophages and interdigitating cells (IDCs) in adult rat thymus after 21 days of treatment, and 21 days after stopping treatment. We also studied the development of IDCs and macrophages in 2, 6, 12, 20 and 30-day-old rats after 21 days of CsA administration to the pregnant mothers. In adult rats after 21 days of CsA treatment, IDCs were absent and only a small number of macrophages were present in the cortex; 21 days after stopping treatment the distribution of IDCs and macrophages had become similar to that in normal adults. The AcP+ macrophages in treated adult rats disappeared, as shown by immunohistochemistry, 21 days after CsA treatment and were again present, similarly to control animals, 21 days after stopping treatment. Therefore CsA causes the thymus medulla of adult rats to disappear and also a significant decrease in the macrophage population. We also found that while in normal rat neonates the thymus has the features of the adult thymus by the 12th day, in neonates from CsA treated mothers this did not appear until the 30th day. CsA treatment to pregnant rats delays thymus development in the young animals but does not cause persisting morphological alterations. This last finding was similar to that observed in adult rats 21 days after the end of CsA treatment.