The hypogonadotrophism model induced by the chronic administration of gonadotrophin-releasing hormone (GnRH) agonist was used to investigate the effects of different concentrations of FSH with or without LH pulses on the stimulation of follicular development in the ewe. Continuous administration of an agonist (buserelin) by osmotic minipump to thirty-six Welsh Mountain ewes from the early luteal phase for 5 weeks resulted in a sustained suppression of the plasma concentration of FSH and inhibited the pulsatile release of LH. The inhibition of gonadotrophin secretion was due to the desensitization and/or down-regulation of pituitary gonadotroph function, since the agonist-treated animals showed no response to a challenge of 1 microgram GnRH. During week 6 of agonist treatment, ewes were infused with either 4-hourly pulses of ovine LH (9 micrograms/pulse), low concentrations of ovine FSH (3 micrograms/h) or high concentrations of FSH (9 micrograms/h) alone or with 4-hourly pulses of LH. After 5 days of gonadotrophin infusion, there was no difference between the mean number of follicles per ewe from the animals treated with LH alone, low concentrations of FSH with or without LH pulses or the high concentration of FSH alone compared with the mean number of follicles from control ewes on day 8 of the luteal phase. Infusion of the high concentration of FSH alone stimulated the development of an increased number of large oestrogenic follicles (follicles greater than 2.5 mm in diameter and secreting greater than 3.7 nmol oestradiol/h in vitro) compared with control ewes. The addition of high-amplitude LH pulses to the infusion of the high concentration of FSH prevented follicles developing beyond 2.5 mm in diameter, but doubled the number of small follicles (less than or equal to 2.5 mm) present in the ovaries. These results show that normal follicular development can be induced by physiological concentrations of FSH alone in the absence of pulsatile LH release. The addition of high-amplitude LH pulses antagonized this stimulatory effect of FSH on follicle growth in the ewe.