To investigate for the first time the effects of ionizing radiation on thymus of a representative cartilaginous fish, juvenile clearnose skates, Raja eglanteria, were exposed to 0-75 Gy of X-radiation and sacrificed after 12 days. Morphometrics (weight, disc width and total length) and thymus and thymic cyst area were compared to controls using ANOVA. Thymus area declined logarithmically and medullary cysts increased as a function of dose (P < or = 0.05). To assess thymic recovery, skates were exposed to 0, 9, 13.5 or 18 Gy of X-radiation and sacrificed when moribund or on days 10, 20, 30 and 40 post-irradiation. Complete restoration of the thymus was not achieved during the 40-day observation period, although repopulation with pro-thymocytes and partial recovery of thymic architecture were evident histologically. The observed high radiosensitivity of R. eglanteria thymocytes was similar to responses of other vertebrates, but recovery time was prolonged.