Gonadotrophin-releasing hormone analogues (GnRH-a) are currently used in combination with gonadotrophins in ovarian stimulation for in-vitro fertilization (IVF). The present study evaluates follicular recruitment and outcome of IVF in patients treated with GnRH-a, starting in different phases of the menstrual cycle. Ovarian quiescence was achieved by s.c. injection of a GnRH-a (600 micrograms/day). Three groups of patients were randomly established. Patients in group 1 (n = 14) started GnRH-a treatment 4-7 days after ovulation. Women in group 2 (n = 15) started GnRH-a 8-10 days after ovulation. Patients in group 3 (n = 15) began the analogue 1-3 days after the onset of menses. Multiple follicular development was achieved by a combination of pure follicle-stimulating hormone and human menopausal gonadotrophin. Oocyte collection was performed 35 h after administration of human chorionic gonadotrophin. Luteolysis was successfully induced in 15% of cases in group 1 and 40% in group 2. Ovarian arrest was achieved in a significantly (P less than 0.01) shorter period of time in group 3 in comparison to groups 1 and 2. There was no difference between groups in the dose of gonadotrophins necessary to reach an optimal response. Patients in group 1 showed a significant decrease (P less than 0.05) in the number and size of follicles developed in comparison to groups 2 and 3. Fertilization and cleavage rates were similar in all three groups. The pregnancy rate was 40% in group 3, while it decreased to 14.3 and 13.3% in groups 1 and 2, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)