Changes in LH secretory granules in pituitary gonadotrophs throughout the sheep oestrous cycle were determined by immunogold localisation of LH at ultrastructural level by electron microscopy. Oestrous cycles in Welsh Mountain ewes were initially synchronised with progestagen sponges and studies carried out in the subsequent cycle. Animals were allocated at random to six groups each of five animals, one killed on day 12 of the luteal phase and the other groups after prostaglandin (PG)-induced luteal regression at PG plus 18 h (early follicular phase), oestrus (PG plus 33·6±1·0 h), oestrus plus 9 h just before the preovulatory LH surge, 1 h after GnRH agonist-induced LH surge at PG plus 48 h (mid-LH surge) and oestrus plus 24 h, after the preovulatory LH surge. Blood samples collected throughout confirmed the pulsatile secretion of LH before and the timing in relation to the preovulatory LH surge. Pituitaries were dissected and processed for transmission electron microscopy and frozen for later extraction of mRNA. Only a single type of LH cell was present in the sheep pituitary. In the luteal phase, LHimmunopositive secretory granules were distributed throughout the cytoplasm in 80% of cells while in 20% of cells granules were polarised to the region of the cell next to a vascular sinusoid. The percentage of polarised cells increased during the follicular phase to 45% at oestrus, 75% at oestrus plus 9 h just before the LH surge and 90% in mid-LH surge. Cell size increased in parallel with polarisation. Gonadotrophs after the LH surge were almost totally devoid of LH granules but prominent LHβ immunoreactivity was observed in the rough endoplasmic reticulum. Analysis of granule diameters revealed a single class of granules with a maximum diameter of 300 nm. Polarised cells had significantly fewer 130–150 nm granules than non-polarised cells, suggesting preferential exocytosis of LH-containing granules of this size from polarised cells. Northern analysis showed that LHβ mRNA levels decreased from luteal through the follicular phase. These results suggest that the preovulatory LH surge in sheep is not related to a change in synthesis of LH but to a progressive recruitment of gonadotrophs into a releasing state, priming, as indicated by polarisation of secretory granules to the region of the cell next to the vascular system.