Clinical trials of norethisterone enanthate (NET-EN), a long-acting, injectable progestogen, have demonstrated its short-term safety and effectiveness as a method of contraception in a controlled trial setting. Family planning programs interested in providing NET-EN need information on its efficacy and acceptability under usual field conditions. This first field study of NET-EN was conducted in family planning clinics in rural areas of Mexico. Women admitted to the study were followed up at two-month intervals to receive an injection of 200 milligrams of NET-EN, and to be asked about possible side effects of the drug. Among the 5,792 women recruited, the overall life-table discontinuation rate was 57.0 per 100 women at 12 months and 69.0 at 18 months. The single most common reason for discontinuation was amenorrhea, followed by bleeding problems. Women who were older and who already had several children were most likely to continue using NET-EN. Nine pregnancies were reported during the study, with a cumulative pregnancy rate of 0.3 per 100 women at 18 months. NET-EN appears to be an effective method of contraception that is acceptable to a substantial proportion of women from rural areas in Mexico who choose to use it, when it is provided through a national family planning program.