Skin reaction to X-irradiation has been studied in the albino quinea-pig; early response in limited-field irradiations of the flank was comparable to that commonly seen in rodents, swine and man, and was dose-dependent with a dynamic range from mild erythema to moist desquamation. The peak early skin reaction was seen between 14 and 21 days after irradiation, and declined before 30 days except at the highest doses used. Fractionation of the X-ray dose at 24 hours resulted in a 'sparing' of about 340 rad. Permanent partial epilation was detectable at doses in excess of 1400 rad, and complete epilation at 1 year occurred in 50 per cent of irradiated fields at 1740 rad. Twenty-four hour two-dose fractionation resulted in a 'sparing' of about 500 rad for epilation. Palpable dermal 'fibrosis' was detectable at 3 months after irradiation in fields given more than 2070 rad, and at 1 year after irradiation in fields given more than 1800 rad; 50 per cent of fields showed palpable 'fibrosis' at 1 year at 1930 rad. Unlike domestic swine and man, skin fields in the quinea-pig showed no dimensional contraction after X-ray doses which produced gross early skin damage.