Abstract: In order to prolong the biological action of norethisterone acetate (Ⅰ) we have synthesized its cyclopentylpropionic ester (Ⅳ). I was reduced to its hydroxy intermediate (Ⅱ) by potassium borohydride in 91% yield and then esterifled with cyclopentyl chloride to give Ⅳ in 49% yield.A single intramuscular dose of 60 mg of cyclopentylpropionic ester of Norethisterone acetate was fo und to protect women from pregnancy for 3 months. The efficacy rate of the contraceptive effect in 1728 cases of clinical trials was about 99.6%.