A chemical method for estimating oestrogens in the urine of pigs (Raeside, 1963) was used to study the levels during the oestrous cycle. Complete and continuous urine collection, free from contamination with faeces, was carried out with two sows for about 4 weeks. The pattern for urinary oestrone showed a well defined peak, near the onset of oestrus, with a relatively sudden fall to levels which remained low until the approach of the next oestrus. The relation of the oestrone peak to oestrus was examined more closely in four sows, with estimations made on urine samples representing 8-hr periods during continuous collection. The findings suggest a preovulatory fall in oestrogen production in the sow. Less reliable estimations were obtained for the oestradiol fraction, and no quantitative results for oestradiol-17β are presented.