ABSTRACT The contents of oestrone and oestradiol-17β were estimated in component parts of sow ovaries representative of the follicular and luteal phase of the oestrous cycle and also in whole juvenile ovaries. The values obtained of the sum total oestrone and oestradiol-17β in the follicular fluid and in the corresponding residual tissue were highly correlated. The proportions of oestradiol-17β tended to be larger in the follicular fluid than in the tissue (mean percentage: 84.3 and 69.6 respectively). In ovaries from the luteal phase of the oestrous cycle the oestrogen levels were generally lower than in ovaries from the follicular phase. Whereas no difference was found between the very low levels of oestrone in the corpora lutea and in the remainder of the ovary containing small follicles, the levels of oestradiol-17β differed significantly, being lower in the luteal tissue. Within each category of ovaries the absolute levels of the oestrogens were very variable. The results are discussed in relation to the pattern of urinary oestrone excretion in the sow during the oestrous cycle.