The release of steroids from Silastic R capsules and rods has been measured in human beings and rats by recovering the implants after periods of subdermal implantation and measuring steroid loss. Reasonably good agreement has been observed between average in vivo release rates in these two species and release rates in vitro . In vivo release rates from capsules frequently showed an appreciable decrease in the first 100 days of use and a more gentle decrease thereafter. In vitro release rates frequently showed a sharp decrease lasting only a few days, but thereafter a moderately declining rate. Rods showed a much steeper and continuing decline in rate. Average measured release rates in human subjects over a one-year period, in μg/day/cm length of capsule, were 3.5 for levonorgestrel, 13.2 for norgestrienone, 17.5 for R2323, 16.4 for megestrol acetate and 8.7 for norethindrone. Calculated expected rates at the end of one year were lower: 2.7 for levonorgestrel, 11.0 for norgestrienone and 12.9 for R2323. Capsules were of 1.57 mm ID and 2.41 mm OD. Significant differences in release rates among subjects using the same drug were observed for levonorgestrel, norgestrienone and megestrol acetate, with the differences being greatest for levonorgestrel. Factors significantly affecting steroid release include the use of gamma irradiation for sterilization and the amount of steroid remaining in the capsules. Gamma irradiation markedly decreased release rate of several steroids through an effect on the steroid. The amount of steroid in the capsule had marked effects on rate in the instance of megestrol acetate, much less effect in the instance of norgestrienone and intermediate effects in the instance of levonorgestrel.