Cyclosporin A (CsA) was administered to rats by repeated subcutaneous (s.c.) or intravenous (i.v.) injections for 14 days. Changes in thymic histology were independent of the route of administration. Blood concentration--time profiles of CsA were similar at dose levels of i.v. 7.5 and s.c. 15 mg/kg/day, respectively. So, provided that the dose is reduced, i.v. injection can serve as an adequate alternative to s.c. injection, thereby preventing unwanted painful side-effects associated with the latter route of administration.