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Effects of the suckling stimulus on the daily LH surge induced by chronic oestrogen treatment were examined in ovariectomized lactating rats. Wistar– Imamichi strain rats were kept under 14 h light:10 h darkness (lights on at 05.00 h). Litter size was adjusted to eight on day 1 (day 0 = day of parturition) and ovariectomy performed on day 2. Lactating rats deprived of their litters on day 0 served as non-lactating controls. Silicone elastomer tubing filled with oestradiol was implanted on day 6 or 15. Blood samples were collected through an indwelling cannula at 10.00 and 17.00 h on each day after implantation to detect daily LH surges. Daily LH surges occurred in the late afternoon in both lactating and non-lactating rats implanted with oestradiol on day 6 or 15. The amplitude of daily LH surges in lactating rats implanted on day 6 declined much more rapidly than in non-lactating rats implanted on day 6, but no significant difference was found in the profile of the LH surge between lactating and non-lactating rats implanted on day 15. Pituitary LH contents just before the daily LH surge (12.00–12.30 h) 4 days after implantation in lactating rats implanted with oestradiol on day 6 were significantly less than those in non-lactating rats implanted with oestradiol on day 6 or 15 and in lactating rats implanted on day 15. These results suggest that the mechanisms responsible for the oestradiol-induced LH surge were not impaired by the suckling stimulus, and that a rapid decline of the amplitude of LH surges observed in mid-lactation could be ascribed to the small amount of LH stored in the pituitary.

J. Endocr. (1988) 118, 311–316

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