SUMMARY The output of oestriol and of pregnanediol during normal pregnancy varies considerably from day to day. This variability detracts from the clinical value of these assays. The variability is reduced if urine collections are made for longer periods of time but such long collections have little clinical application. An attempt was made to reduce the high variability of 4 hr. urine collections by standardizing conditions such as rate of urine secretion, posture and activity. The results of such 4-hourly 'steady state' collections were compared with the results obtained from corresponding 24 hr. and 48 hr. collections. It was concluded that under standardized conditions the 4 hr. collection was very little more variable than the 24 hr. one and gave comparable results.