Measurement of immunoreactive progesterone, pregnanediol and oestradiol in faeces collected throughout ovarian cycles in three species of callitrichid primates is reported. Faecal hormone concentrations were compared with plasma progesterone profiles during PGF2 alpha-controlled (n = 7) and natural (n = 8) cycles in Callithrix jacchus and Saguinus fuscicollis, respectively, and with urinary oestrone conjugates during five cycles in Saguinus oedipus. Unconjugated steroids, which predominated over enzyme hydrolysable conjugates in samples from all species, were used to generate cycle profiles. According to results from HPLC, oestrone and oestradiol accounted for virtually all oestrogen immunoreactivity, and oestradiol most often predominated, whereas large amounts of nonspecific immunoreactivity were detected by both progesterone and pregnanediol assays. Faecal progestins were excreted in a cyclic manner in all species; luteal phase values were on average five- to tenfold higher than corresponding follicular phase values. Significant increases in mean amounts of faecal progestins were seen within 48 h of the post-ovulatory rise in plasma progesterone. Although a similar trend was also seen for faecal oestradiol, a clear and consistent luteal phase increase was seen only in Callithrix jacchus and this generally occurred later than that of progestins. The results indicate that faecal progestin analysis provides a useful method for noninvasive reproductive assessment in callitrichid primates. In particular, measurement of immunoreactive pregnanediol enables a multispecies application of a single assay methodology for comparative studies on callitrichid reproductive function.