Two unanaesthetized female yellow baboons (Papio cynocephalus cynocephalus) were infused (i.v.) with [3H]oestradiol and two with [3H]progesterone, early in the follicular phases of their cycles. One month later, the two females infused with [3H]oestradiol were simultaneously infused with [14C]progesterone and [3H]dehydroepiandrosterone. All urine and faeces were collected for 96 h after infusion. The proportion of steroid excreted in faeces (versus urine) was 10.0% for oestradiol and 40% for progesterone. Peak excretion in urine occurred 4.5 h after infusion. Peak excretion in faeces occurred an average of 36.4 h after infusion, with remarkable consistency between steroids. Eighty per cent of faecal oestradiol and progesterone metabolites were excreted as free (rather than conjugated) steroids. Simply boiling (20 min) the dried faecal sample in 90% ethanol proved to be the most rapid and efficient means of extracting these steroid metabolites. High pressure liquid chromatography and immunoreactivity studies revealed that oestradiol was excreted in faeces as oestradiol (36%), oestrone (44%) and a conjugated metabolite that co-eluted with oestrone sulfate (20%). Progesterone was excreted as eight different free forms, only a minor portion of which was progesterone, and what appeared to be a conjugated metabolite that co-eluted with pregnanediol-glucuronide (20%). The free progesterone metabolites were identified by gas-chromatography-mass-spectrometry as epimers of 5-pregnane-3-diol and 5-pregnane-3-ol-one. These data suggest that currently available immunoassays for free oestradiol and oestrone should adequately characterize faecal oestrogen profiles in baboons. However, high variability in crossreactivities of various progesterone antisera to progesterone metabolites in baboons makes antiserum selection a more serious concern in attempts to quantify faecal progestogen dynamics.(ABSTRACT TRUNCATED AT 250 WORDS)