Twelve anoestrous ewes maintained under natural photoperiod at 57°N received an oral dose of 3 mg melatonin daily at 15:00 h from 1 May. Starting 41 days later and extending from 11 June until 5 September, six of the ewes were also infused continuously with 0.8 mg thyrotrophin-releasing hormone (TRH) day−1 via subcutaneous osmotic minipumps. The remaining six ewes acted as controls. Behavioural oestrus, ovulation rate and luteal function were determined by exposure to a vasectomized ram, laparoscopy and the measurement of progesterone in peripheral plasma, respectively. TRH infusion stimulated a sustained increase (P < 0.001) in plasma concentrations of thyroxine, tri-iodothyronine and prolactin (thyroxine: 158 ± 9.3 and 65 ± 7.7 nmol l−1 for TRH-infused and control ewes, respectively; tri-iodothyronine, 2.6 ± 0.12 and 1.1± 0.19 nmol−1 and prolactin, 57±12 and 11 ± 2 μg l−1). No ewes were in oestrus before the TRH infusion and the mean number of behavioural oestrous cycles per ewe during the infusion period was 1.3 ± 0.33 and 2.5 ± 0.34 for TRH-infused and control ewes, respectively (P < 0.05). Corresponding mean intervals from 1 May to the onset of the first luteal phase (progesterone > 1 ng ml−1) were 88 ± 8.9 and 79 ± 3.5 days (not significant). TRH infusion had no effect on the mean numbers of corpora lutea (1.7 ± 0.14 and 1.6 ± 0.20 for TRH-infused and control ewes, respectively), but was associated with a lower mean incidence of normal luteal phases (1.5 ± 0.43 versus 2.7 ± 0.21, P= 0.052). Abnormalities in luteal function included delayed initial expression, extended ovarian cycles, suprabasal periovulatory progesterone concentrations and protracted periods of low progesterone secretion between successive ovarian cycles. Thus continuous TRH infusion suppressed plasma prolactin, doubled the circulating concentrations of thyroxine and tri-iodothyronine, and was associated with a wide range of abnormalities in ovarian function and endocrine status, the nature of which varied between ewes.