Patterns of thymic regeneration in rats after single or divided doses of cyclophosphamide

Fractional doses of cyclophosphamide (Cy; 5 X 60 mg per kg) given to rats caused more severe initial destruction of thymic cortex and medulla, with a corresponding decrease of thymic mass, than did a single dose of the same amount (300 mg per kg). After the fractional dose, regeneration was retarded and repopulation was suppressed, so that on day 15 the thymus was almost totally alymphoid, in contrast to the initial regeneration seen after the single dose. On day 20 after the fractional dose of Cy an initial degree of regeneration corresponding to that seen on day 15 after the single dose, was found. The delayed restriction of regenerative capacity was more marked after the divided dose than after the single dose of Cy.