The effects of oestrogen priming and GnRH administration on the release of LH and on luteal function were investigated using 66 spring-lambing S.A. Mutton Merino ewes in early lactation. Seven randomly selected groups of ewes received either (i) 30 μg oestradiol benzoate (OOB) divided into three injections followed by 50 μg GnRH injected in two equal doses, (ii) 30 μg ODB as three injections, but not followed by GnRH, (iii) 50 μg GnRH as two injections without ODB pre-treatment, (iv) a single injection of 30 μg ODB as a prelude to 50 μg GnRH in two injections, (v) 30μg ODB not followed by GnRH, (vi) 30μg ODB in one administration followed by a single dose of 50 μg GnRH or (vii) remained untreated and served as controls. The LH concentration of jugular blood samples obtained every 30 min for 12 hours after the time of the first GnRH injection was determined. The best response in terms of, the proportion of ewes exhibiting an LH release, the greatest concentration, the total quantity released and the duration of LH release, was obtained where both the oestrogen priming and releasing hormone stimulation were administered as divided doses (Treatment (i) In contrast, all these characteristics of the LH release were significantly lower in the ewes treated only with three doses of oestrogen (Treatment (ii) and where a single injection of GnRH followed 30 μg ODB, also as a single dose (Treatment (vi) In the latter treatment the peak LH level was not significantly lower than in treatment (i). A further significant reduction in the characteristics of the LH surge was noted in the remaining treatment groups (Treatments (iii), (iv) and (v) The LH release in the ewes which received only a single 30 μg dose of ODB (Treatment v) was so delayed that only part of the release curve could be quantitated and then only in 55,6 per cent of the ewes treated. Plasma concentrations of progesterone at 2-4 day intervals after the LH release demonstrated that all the treatments resulted in unexpectedly poor luteal function. The treatments applied were not capable of precipitating the onset of oestrous cycles in ewes which were in early lactation during spring.