1. The minimum dose of x-ray, of each of three degrees of hardness, required to produce persistent alopecia of the abdominal skin of normal white rabbits has been studied. 40 kilovolt rays filtered by 0.5 mm., 50 kilovolt rays filtered by 1 mm., and 70 kilovolt rays filtered by 1.5 mm. of aluminium were employed. The 70 kilovolt work is unfinished.
2. Under constant conditions of treatment, the animals were found to react with a high degree of consistency.
3. The values of the critical minimum doses expressed in terms of air ionization (pair of ions per gm. of air divided by 1015), for rays produced at 30, 40, 50, and 70 kilovolt peak, are 2.04, 3.7, 4.5, and over 5.1 respectively. The 30 kilovolt value is taken from a previous paper.
4. It is shown that the alopecia reaction does not run parallel to the erythema and pigmentation reactions.