Previous reports by other investigators on the presence of oestrone and oestradiol-17β in stallions' urine have been substantiated by chemical methods including infrared spectrophotometry. In addition a third oestrogen, oestradiol-17α, was isolated from this source. Quantitative chemical determinations of oestrogens in urine samples from 36 stallions from 7 to 37 months old, showed that only insignificant quantities were excreted during the first two years of life. At this time there was a marked increase, with oestrone predominating. In 8 mature stallions the average value for oestrone was 20.7 mg per litre of urine. Both oestradiol-17β and oestradiol-17α were excreted in amounts approximating 1/10 of this value. Using the same method only trace amounts of urinary oestrogens were recorded in 10 fully grown geldings.