The effects of time postpartum and suckling status on the oestradiol-induced LH surge in ewes and some of the possible mechanisms involved were investigated. Lambs were removed from nonsuckled ewes at parturition (day 0) and the ewes hand-milked 12 and 48 h later to prevent discomfort. Nonsuckled ewes on day 1, 7, 14 or 28, and suckled ewes on day 7 or 28 postpartum were injected i.m. with 50 micrograms oestradiol benzoate (hour 0). A further eight suckled ewes were given oestradiol on day 1 (as above), and four were also given naloxone (1 mg kg-1 h-1) from 10 until 14 h after administration of oestradiol benzoate. There was no LH surge in response to oestradiol on day 1. In nonsuckled ewes the onset of the LH surge was significantly (P < 0.05) delayed when oestradiol was administered on day 7 (17.4 +/- 1.5 h), compared with when it was injected on days 14 and 28 postpartum (14.4 +/- 1.7 and 14.0 +/- 1.4 h). The LH surge was also delayed (P < 0.05) when oestradiol was administered on day 7 (18.0 +/- 2.8 h) in suckled ewes. There was no difference in the duration or amplitude of the LH surge with time postpartum in nonsuckled ewes. Suckling significantly (P < 0.05) reduced the amplitude of the LH surge on both day 7 (21.0 +/- 2.4 versus 26.4 +/- 2.3 ng ml-1) and day 28 (23.6 +/- 3.2 versus 28.8 +/- 2.9 ng ml-1), but had no effect on the onset or duration of the surge.(ABSTRACT TRUNCATED AT 250 WORDS)