Mean diffusion of 6-methyl-16-methylene-17 α-acetoxypregna-4, 6-diene-3, 20-dione through dimethylpolysiloxane membrane into water was 140 μg/24 hrs/100 mm 2/0.1 mm; (dl)-13β-ethyl-17 α-ethynyl-17β-hydroxygon-4-en-3-one diffused with a rate of 73 μg/24 hrs/100 mm 2/0.1 mm. When plasma instead of water was used for incubation, the diffusion rates were increased by the following factors: 2.4 times for 19-norpregn-4-ene-3, 20-dione; 1.5 times for 6-methyl-17 α-acetoxypregna-4, 6-diene-3, 20-dione, and 4.7 times for (dl)-13β-ethyl-17 α-ethynyl-17β-hydroxygon-4-en-3-one. Measurements of diffusion through porous glass discs, and of solubility in different solvents indicated that the steroid molecules diffuse through the dimethylpolysiloxane in a non-solvated form.