The influence of the dose, as well as the application area of topically applied 3H-estradiol in a volatile solution on its systemic bioavailability was investigated in hairless rats in vivo. The bioavailability was determined by comparing the urinary and fecal excretion of radioactivity after intravenous injection and after topical application. Whatever the size of the surface of application (1, 4 or 16 cm2), the bioavailability was similar (47–65%) 4 days after the application of a single low dose (50 nmol). When the surface of application was washed 24 h after dosing, the absorption was the same for the areas of 3 and 9 cm2. In all cases average fluxes on the first day were higher for the larger area. When applied doses were increased (1,000 and 10,000 nmol), the percentage of percutaneous absorption decreased with the reduction in the areas of application. Thus, with a dose of 1,000 nmol applied on a surface of 1, 4 and 16 cm2, absorption was equal to 18.7, 21 and 37%, respectively, of the dose. There was a linear relationship between the log of the dose applied per unit of skin area and the percentage of absorption after 4 days (r = 0.99 with washing; r = 0.98 without washing). For a determined dose of estradiol (D) it is thus possible to predict the total quantity (Q) which will be absorbed depending on the application area (A) according to the relationship: Q=(-alogD/A + b)/100 ×D, where a and b are equal to 18.7 and 73.8, respectively, for an application without washing and 20.4 and 68.8 with washing. From these results it appears that when estradiol is applied in a volatile solvent at low concentration the horny layer acts as a reservoir. After evaporation of the vehicle and for a single dose the bioavailability is equivalent for a large range of application areas. Thus, with a volatile solvent it would not appear necessary to control both the dose and the surface of application in order to obtain controlled dosage for systemic effec