Eleven unconjugated steroids were measured daily during a complete cycle in the peripheral plasma of 6 normally menstruating baboons (Papio hamadryas) by means of a radioimmunoassay procedure and the levels were compared with those found previously in 17 normally menstruating women. The patterns of progesterone and 20alpha-dihydroprogesterone were very similar to those found in women throughout the entire menstrual cycle. However, the ratio of these steroids differed markedly from that found in women. A great similarity of the follicular phase and the peri-ovulatory period profiles was observed in both species for 17-hydroxy-progesterone, pregnenolone, androstenedione, oestradiol and oestrone. The pattern of oestradiol:oestrone ratios was similar in both species. However, an elevation of the above five steroids, typical for the human luteal phase, was not found in baboons. The increase of testosterone values, seen in women at mid-cycle, was not detected in baboons. The plasma concentrations were lower in baboons than in humans for all the above steroids.