Various reports (1-6) have characterized the gross effects resulting from both single and periodic external exposures to 3-radiation in laboratory animals and man. This report is one of a series of investigations designed to impart information regarding dose-effect relationships of skin subjected to f-radiation and the gross pathology of skin response to such treatment. The relationship between total dose and time required to produce tissue breakdown has been demonstrated to be essentially dose-independent for a total dose up to 20,000 rads, occurring at 9 to 11 days postexposure (1). The threshold for wet desquamation of the skin of the rat (1), the pig (2), and man (5, 6) is approximately 2000 rads; the guinea pig (1) and the rabbit (3) require much higher doses to elicit this same effect. When the total dose is divided into fractional parts and given at intervals, the dose required for an equivalent effect is increased. Various investigators have utilized the erythema or pigmentation response as an index of the degree of damage incurred (4-8). We are concerned here primarily with a consideration of the damage to skin induced by ionizing radiation as separate from the deep effects associated with more penetrating radiation such as X-rays, y-rays, and fast neutrons. Moreover, rather than rely on either the sharp erythema reaction as did Reisner (9) or the threshold erythema dose of Duffy et al. (10), we set about first to establish a standard reaction which would be applicable in the case of the rat. Erythema may be the earliest reaction or gross response to radiation skin damage (6), but it most certainly cannot be used as a reliable criterion of injury in the rat. At low doses (below 2000 rads) it is fleeting, inconstant, and, more often, indistinct. Above 5000 rads, it is definitely recognizable; it occurs early and persists until more severe injury becomes apparent (a matter of 3 to 7 days, depending on the severity of the dose, i.e., 5000 to 50,000 rads) (1). Pigmentation, used by MacComb and Quimby (7) as a standard threshold skin dose in human studies in preference to the more transient threshold erythema dose, is of no use in the albino rat for obvious reasons.