When the skin of the shoulder ("A" field) and lower back ("C" field) is irradiated through 10-cm-diameter fields with 250-kVp x-rays, having a HVL of 0.87 mm copper, a dose range is reached between approximately 1600 rads and 3000 rads in which a moist reaction is or is not formed. If a moist reaction is formed, it either heals completely, partially, or not at all. The evolution, time course, and dose dependence of the moist reaction that occurs following irradiation has been determined. The moist reaction is found at 17.5 +/- 0.6 days in the "A" field, and 20.8 +/-0.8 days in the "C" field. The reaction evolves to involve from 5 % to 100% of the field by 24.9 +/- 0.5 days in the "A" field and by 28.5 +/- 1.0 days in the "C" field. Healing is complete by 36.0 +/-1.0 days in the "A" field and by 38.0 +/- 1.3 days in the "C" field. The area of the field involved with a moist reaction at the time of maximal involvement is dose-dependent. The area of the field involved with a moist reaction at the time of complete healing is also dose-dependent. The dose at which 50 % of the fields were not healed was 2273 +/-103 rads in the "A" field, 2578 +/-141 rads in the "C" fields, and 2437 +/- 89 rads in the "A" and "C" fields. The values in the "C" field are significantly different from those in the "A" field except for the dose at which 50 % of the fields were not healed and the time at which the field was maximally healed.