Plasma luteinizing hormone (LH) concentrations were measured in normal (control) Corriedale x Merino (comeback) ewes and in clover-infertile comeback ewes which had grazed oestrogenic Yarloop clover (Trifolium subterraneum L. cv. Yarloop) for more than 4 years. Plasma LH concentrations were measured in samples taken at 20-min intervals for 6 h during the dioestrous stage of the oestrous cycle in the breeding season (BS) and during the anoestrous season (AS). In the control ewes during BS, transitory elevation in plasma LH concentration (pulses) occurred, reflecting secretory episodes, with a frequency of one per 5·2 h. This frequency fell to one per 16·5 h during the anoestrous season. In clover-infertile ewes, LH pulses occurred with a frequency of one per 4·5 h during BS and one per 4·9 h during AS (difference not significant). In the controls, plasma LH levels were higher (P<0·05) during BS (mean ± s.d. = 1·2 ± 0·4 ng/ml, n = 9) than in AS (0·7 ± 0·3 ng/ml, n = 5). In the clover-infertile ewes, plasma LH levels in BS (1·3 ± 0·6 ng/ml, n = 12) were similar to those of controls. During AS, plasma LH levels in the cloverinfertile ewes (1·0 ± 0·6 ng/ml, n = 10) remained similar to their BS levels, being significantly (P<0·05) higher than LH levels in the controls at this time. These studies indicate that the higher plasma concentrations of LH which have been reported in clover-infertile ewes arise from more frequent LHpulses. Furthermore, in contrast to normal ewes, average plasma LH, reflecting pulse frequency, is not reduced in AS. This supports the view that ingestion of phytooestrogens affects neural centres involved in regulating LH secretion.