SERUM NORETHINDRONE LEVELS AND MENSTRUAL PATTERN AFTER THE USE OF NORETHINDRONE IMPLANTS IN RHESUS-MONKEYS

Subdermal Silastic implants releasing norethindrone (NET) were inserte d in five rhesus monkeys for a period of seven months. The total lengt h of the implants used was 160 mm (eight implants of 20 mm each) conta ining 160 mg of NET (1 mg NET/mm). The in vivo release rate of NET fro m the Silastic implants containing 160 mg NET was 3.12 mu g/day/mm. Me nstrual bleeding records were maintained and serum NET levels were est imated from blood samples collected during the period implants remaine d in situ. There was an initial rise in NET levels, followed by fluctu ating levels of NET in three out of five monkeys. The menstrual cycle or bleeding episodes showed a pattern commensurate with the circulatin g NET levels. None of the monkeys showed intermenstrual bleeding durin g the treatment period. High levels of NET seem advantageous for an un disturbed menstrual cycle.