To determine the timing of onset of contraceptive effectiveness after the first injection of 150 mg of depot medroxyprogesterone acetate (DMPA) administered between days 8 and 13 of the menstrual cycle. Descriptive, prospective study. A tertiary university referral center. Thirty healthy women between 18 and 40 years of age. Volunteers were injected with DMPA between days 8 and 13 (5 women on each day) of the menstrual cycle. Ovarian function determined by serum levels of E2 and progesterone and follicular development evaluated by vaginal ultrasound. In nine (30%) of 30 women studied, DMPA did not prevent ovulation. All ovulations occurred in women receiving DMPA between days 10 and 13 of the cycle. No woman who received injections on day 8 or 9 ovulated. Ovulation suppression was more effective in women with low ovarian activity. All ovulation occurred within 3 days after the injection. A back-up contraceptive method, used after the 7th day of the menstrual cycle, is recommended for up to 7 days after the first injection of DMPA.70817-20