The effect of the reduction of prolactin (PRL) release by the bromocriptine (CB-154) treatment on suppression of luteinizing hormone (LH) pulses by the suckling stimulus was examined in ovariectomized lactating rats at mid-lactation. Litter size was adjusted to 8 on day 1 (day 0=the day of parturition). Postpartum rats deprived of their pups on day 2 served as non-lactating controls. All rats were ovariectomized on day 2. CB-154 (0.6 mg/day) or saline was injected daily into both lactating and non-lactating rats from day 2. Ovine PRL (0.3 mg/day) was infused with a mini-osmotic pump into the half of the animals treated with CB-154. Litters were rotated every day among a CB-154 treated mother, 2 intact mothers and a saline-injected mother to ensure the similar strength of the suckling stimulus. Blood samples were taken at 6-min intervals for 3 hr on day 7 or 8. Pulsatile LH secretion was strongly suppressed in all lactating rats in spite of the treatment of CB-154. Frequent LH pulses were observed in all non-lactating animals, suggesting that CB-154 or ovine PRL did not directly affect LH secretion at the doses employed. These results suggest that PRL does not mediate the suppressing effect of the suckling stimulus on pulsatile LH secretion in rats at mid-lactation.