The early and late effects of X rays on lymphatic microcirculatory function have been investigated in pig skin. Paired estimates of lymph clearance were performed in irradiated and unirradiated sites on the flank skin of anaesthetized Large White pigs at 3, 6, 9, 12, 26, 39, 52, 64 and 78 weeks after a single dose of 18 Gy of X rays. The dermal clearance of 99Tcm-colloid was used to assess lymph flow. The results demonstrated two waves of impaired lymphatic clearance which correlated well with the gross morphological changes observed. The first wave occurred between 6 and 12 weeks after irradiation and corresponded to a period of ischaemia and oedema. The second wave, at 52 weeks, corresponded with dermal thinning and subcutaneous atrophy. X irradiation resulted in an impairment of lymph drainage both within and outside the field of irradiation. Impaired lymphatic drainage probably contributes to the gross and histological changes observed in the skin after X irradiation.