Concentrations of oestrogens*, progesterone and 17α-hydroxyprogesterone were determined in daily peripheral plasma samples during one normal menstrual cycle in eight rhesus monkeys. The oestrogens were measured by a rapid radioimmunoassay. Progesterone and 17α-hydroxyprogesterone were determined by competitive protein binding techniques subsequent to column separation on hydrophobic Sephadex. Oestrone and oestradiol in pooled plasma samples were determined by radioimmunoassay after column chromatography. The plasma concentration of oestrogen showed a gradual late follicular rise to a midcyclic peak followed by an abrupt fall for 1–2 days and a subsequent small rise to concentrations similar to those preceding the mid-cyclic peak. Plasma levels of progesterone were below 1 ng per ml before the mid-cyclic peak of oestrogens but rose significantly one day after the oestrogen peak, and reached a luteal plateau (range 3.4–11.3 ng per ml) five to six days later. The plasma levels of 17α-hydroxyprogesterone increase paralleled the mid-cyclic peak of oestrogens. The luteal phase pattern of 17α-hydroxyprogesterone mirrored the progesterone pattern, but the concentration was somewhat lower than for progesterone. All three steroid hormones measured decreased prior to the onset of the menstrual bleeding. Oestradiol (2.4–6.9 ng per 100 ml) dominated over oestrone (1–3.2 ng per 100 ml) in the follicular phase while the reverse was true during the luteal phase. The ratio of E2:E1 was 2:1 or higher in the follicular phase, but during the late luteal phase the ratio was reversed. The patterns of the three steroid hormones observed during the menstrual cycle were qualitatively similar to those reported in women, but quantitatively the oestrogen and progesterone levels were lower, while the levels of 17α-hydroxyprogesterone were higher in the rhesus monkey.