Previous studies of exposure of normal skin to ultraviolet radiation have demonstrated a cumulative effect lasting greater than 24 h when repeated suberythemal exposures are given. However, the time course of recovery from a single suberythemal dose of ultraviolet A (UVA) or ultraviolet B (UVB) has not been determined. We show here for the first time that the period required for recovery of normal skin (as measured by delayed erythema) following a single suberythemal dose of UVA is between 30 and 48 h, and for UVB is between 24 and 30 h. Photoprotection was noted for both UVA and UVB from the fifth through the ninth day after the single suberythemal exposure, but was only statistically significant on the fourth day after UVB exposure. The curve depicting recovery from a single suberythemal dose of UVA or UVB from the first irradiation time through the fourth day after irradiation may be described as an exponential decay curve. Formulas are given for both UVA and UVB which describe the exponential nature of this curve. These formulas may be used to predict the exact difference in erythema threshold between preirradiated and normal skin. From the fourth day after exposure to the ninth day, the curve is nearly constant. The nature of the recovery curve in the first 4 days after exposure suggests that an exponential decay process occurs in UVA or UVB damage, consistent with unstable photoproduct decay, DNA repair, or constitutive enzymatic processes.