PIP: Drug release from Chinese subcutaneous implants containing levonorgestrel was studied. It was compared with that of Norplant. A horizontal vibration method was applied in this experiment, all conditions in keeping with the requirements of the "sink condition." The daily release of levonorgestrel in vitro was assayed by measuring the UV absorption of the solution at 240 nm. The results from 2 years of release experiments in vitro showed that the release rate of levonorgestrel from the Chinese implant was constant with good reproducibility, and no bursting effect could be seen. The Chinese implants release levonorgestrel with order and the average daily release was 85 mcg. Norplant had a 10 day bursting effect before the release rate became constant. The daily release of levonorgestrel was 68 mcg. (author's modified)