Serum norethindrone levels and menstrual pattern after the use of norethindrone implants in rhesus monkeys

Subdermal Silastic implants releasing norethindrone (NET) were inserted in five rhesus monkeys for a period of seven months. The total length of the implants used was 160 mm (eight implants of 20 mm each) containing 160 mg of NET (1 mg NET/mm). The in vivo release rate of NET from the Silastic implants containing 160 mg NET was 3.12 micrograms/day/mm. Menstrual bleeding records were maintained and serum NET levels were estimated from blood samples collected during the period implants remained in situ. There was an initial rise in NET levels, followed by fluctuating levels of NET in three out of five monkeys. The menstrual cycle or bleeding episodes showed a pattern commensurate with the circulating NET levels. None of the monkeys showed intermenstrual bleeding during the treatment period. High levels of NET seem advantageous for an undisturbed menstrual cycle.