SUMMARY The concentrations of progesterone, oestrogen and luteinizing hormone (LH) in ovarian venous plasma, and of progesterone, follicle-stimulating hormone, LH and prolactin in peripheral plasma were measured in 47 Welsh Mountain sheep at accurately timed stages around oestrus. The histology and ultrastructure of the pre-ovulatory and newly ruptured follicles in these animals were also examined. The highest ovarian vein levels of oestrogen were invariably obtained from the ovary containing the largest non-atretic follicle. A significant amount of oestrogen was already being secreted by day 13, when both the peripheral and ovarian venous blood progesterone levels were still high. On day 15 the corpus luteum stopped secreting progesterone; coincident with this decline there was a maximal increase in oestrogen secretion from the largest non-atretic follicle. The thecal cells of this follicle enlarged concomitantly with the rise in oestrogen, and maximal development of the smooth endoplasmic reticulum was observed at or after the oestrogen peak. By 6 h after the onset of oestrus, progesterone concentration was still very low, and oestrogen values had fallen in most animals, while LH showed a pronounced increase. The maximal level of prolactin was also found at this time. By 15 h after the onset of oestrus, both progesterone and oestrogen values were low and LH had returned to baseline levels except in a few animals. Ovulation had occurred in 11 out of 12 animals by 30 h after the onset of oestrus and the recently ruptured follicle was just beginning to secrete a little progesterone. Luteinizing hormone and oestrogen values were very low.