Mechanisms responsible for suppression of FSH and LH secretion during lactation were investigated in rats, with special reference to the suckling stimulus and ovarian inhibin. Concentrations of immunoreactive inhibin in the peripheral plasma and bioactive inhibin in ovarian venous plasma were always low on days 3 and 5 of lactation in dams nursing eight pups, whereas values were always high on days 17 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups. There was an FSH surge within 48 h after removal of litters on days 3 and 5 of lactation in dams nursing eight pups, whereas plasma concentrations of FSH were unchanged within 48 h by removal of litters on days 17 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups. Plasma LH concentrations increased significantly compared with those of control animals within 24 h after removal of the litter on any day of lactation, regardless of the litter size. Plasma FSH levels increased within 6 h after bilateral or unilateral ovariectomy in lactating rats only on the days when plasma concentrations of inhibin were high before ovariectomy, such as day 17 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups, whereas the mean concentrations of plasma LH showed no significant increase within 12 h after bilateral ovariectomy in these lactating rats. Treatment with progesterone or oestradiol-17 beta after unilateral ovariectomy did not inhibit the increase in plasma FSH levels, while the increase in plasma concentrations of FSH after surgery was completely inhibited by injecting inhibin (porcine follicular fluid). Treatment with steroid hormones inhibited the basal levels of LH in unilateral ovariectomized lactating rats. Plasma FSH concentrations increased sharply within 6 h after a single i.v. injection of anti-inhibin serum on days 10, 15 and 20 of lactation in dams nursing eight pups and on day 5 of lactation in dams nursing two pups, whereas only a small but significant increase in concentrations of FSH was noted 6 h after the antiserum treatment on day 5 of lactation in dams nursing eight pups. Concentrations of plasma LH were unchanged by treatment with antiserum in lactating rats throughout lactation.(ABSTRACT TRUNCATED AT 400 WORDS)