Two groups of female rhesus monkeys, one consisting of five normally menstruating animals and the other comprising three animals in the last month of pregnancy, were used to follow the urinary excretion of oestrogens. In view of the observations of Short & Eckstein (1961) that oestriol appeared to be absent in the urine of pregnant monkeys it was decided to study the urinary excretion of oestrone, oestradiol-17β and oestriol in both non-pregnant and pregnant monkeys. Brown's method (Brown, 1955) for the estimation of oestrone, oestradiol-17β and oestriol was adapted to monkey urine. It consisted of hydrolysis with HCl of a 24-hr. sample of urine, extraction and separation into oestrone plus oestradiol-17β (fraction I) and oestriol (fraction II) and methylation of the two fractions as described by Brown (1955). The methylated fractions I from 24-hr. urine samples of 3 consecutive days, treated as described above, were pooled and chromatographed on alumina