More accurate knowledge of the time of onset of cervical mucus changes after Depo-Provera injection would enable family planning providers to counsel new acceptors to use a backup method only for the amount of time it is really needed. To obtain such data, 30 women from a family planning clinic in Campinas, Brazil, who requested Depo-Provera in 1995-96 were recruited. At baseline, 11 women had a poor cervical mucus score (0-4), 12 had a fair score (3-8), and 7 scored in the good range (9-12). Between 6 and 24 hours after injection, a sharp decline occurred in the cervical mucus score in all but 3 patients (each of whom had a "good" score at baseline). On day 3, 29 women had "poor" mucus and the remaining woman had a cervical mucus score in the fair range. By day 7, 29 women had zero mucus scores; the score in the last woman was 1. 24 hours after injection, sperm penetration (measured by vanguard sperm distance) had decreased to under 1 cm in all but 4 women. At day 3, 1 of the 2 women with good penetration at 24 hours maintained a vanguard sperm distance of 2.5 cm and an estimated sperm count of 15,363. By day 7, sperm penetration was zero in all but 1 woman (0.5 cm). These findings confirm that depot medroxyprogesterone acetate causes profound changes in cervical mucus after injection. The lack of a more complete hostility to sperm penetration at day 3 was unexpected, however. Given the nonavailability of a reliable clinical marker to verify a contraceptive effect at 3 days, it seems prudent to advise new Depo-Provera acceptors to use a backup method for the first 7 days.