The efficacy of a recently engineered single chain recombinant equine follicle stimulating hormone (reFSH) was investigated in estrous cycling mares whose gonadotropins and follicular activity had been suppressed by concurrent treatment with progesterone and estradiol (P&E). Time of estrus was synchronized in 15 estrous cycling mares during the breeding season with prostaglandins F(2alpha) (PGF(2alpha)). The day after ovulation, mares were treated once daily with P&E for 14 days. Mares received a second injection of PGF(2alpha) on day 6 of the synchronized estrous cycle to induce luteolysis. On day 8 post-ovulation mares were randomly assigned to three groups: small dose reFSH-treatment group (0.5mg reFSH IV, twice daily); large dose reFSH-treatment group (0.85mg reFSH IV twice daily); control group (saline IV, twice daily). reFSH treatment occurred concurrently with the last week of P&E treatment. After a follicle or cohort of follicles reached 35mm in diameter, mares were injected with 0.75mg of recombinant equine luteinizing hormone (reLH) to induce ovulation. Post-treatment ovulation was assessed. Daily blood samples were collected for analysis of FSH, LH, estradiol, progesterone, and inhibin by radioimmunoassay (RIA). On the first day of reFSH/saline treatment, blood samples were collected periodically from 1h prior to treatment to 6h post-injection via an indwelling jugular catheter to determine acute changes in FSH concentrations. Monitoring of follicular activity, estrus, and ovulation was performed daily by utilizing a stallion and transrectal ultrasonography. A difference (p<or=0.05) between the largest diameter follicle in the reFSH-treatment groups compared to controls occurred on day 14 post-ovulation, the day treatments ended, and the difference continued until day 21 post-ovulation. reFSH-treatment groups had larger (p<or=0.05) numbers of 20-29mm follicles (days 13-18), 30-34mm follicles (days 15-20) and >or=35mm follicles (days 16-21) than controls. Mares treated with reFSH, at either dose, took less time (average: 2.95+/-0.42 days) to develop 2-3 times more pre-ovulatory follicles than control mares (7.8+/-0.51 days) (p<or=0.05). The number of ovulations between treated mares and controls were similar due to a greater incidence of ovulation failure in reFSH-treated mares. During reFSH treatment, concentrations of plasma FSH, inhibin and estradiol were greater (p<or=0.05) compared to control concentrations. Plasma LH concentrations in reFSH-treated mares were suppressed and did not exhibit the ovulatory surge of controls (p<or=0.05). Plasma progesterone concentrations were not different across groups. These findings demonstrate the specific effects of reFSH to increase number of total follicles including pre-ovulatory follicles in mares with endogenous pituitary gonadotropins and follicular growth suppressed by a regimen of P&E.