Determination of radioactivity in urine and faeces was used to monitor absorption of labelled 6-methyl-17α-acetoxypregna-4,6-diene-3,20-dione from polydimethylsiloxane implants. Based on previously determined in vitro diffusion rates, mean absorption was 60% in hamsters, 80% in rabbits, and 85% in rats during the first month after implantation. Thereafter, excretion of radioactive material has slowly decreased. After six months, the rate declined to 40% of the original value.