Methotrexate (MTX) has the potential of eradicating small infiltrations of leukemic cells in the gonads. We examined the morphological changes in rabbit testes after a single dose (57.5 mg/kgBW) or repeated low doses (6 mg/kgBW once a week for 14 weeks) of MTX IV. Fertility rate and spermatogenic activity were evaluated using the tubular fertility index (TFI). Testicular MTX concentrations (measured by RIA) were in the same range in both groups. Only after repeated MTX application were reduction of TFI and signs of germinal cell line degeneration found. When given repetitively, MTX therapy may modify the fertility and tubular morphology. Long-term follow-up will show how these changes affect future fertility.