ABSTRACT Supernatant preparations of armadillo placental homogenate were incubated with testosterone-4-14C, androst-4-ene-3,17-dione-4-14C, dehydroepiandrosterone-4-14C, 16α-hydroxydehydroepiandrosterone-7α-3H and pregn-5-enolone-4-14C, using Ryan's classical techniques for tissue preparation and incubation. Evidence based on paper chromatography, methylation, thin layer chromatography and acid fluorescence, in conjunction with reverse isotope dilution, shows that oestriol was formed in vitro when 16α-hydroxydehydroepiandrosterone-7α-3H was used as the precursor. The other androgenic precursors were converted to phenolic compounds to a much smaller degree, possibly to oestrone and 17β-oestradiol. No aromatization appeared to take place when pregn-5-enolone-4-14C was studied as a precursor for oestrogens. Pooled armadillo pregnant urine and pooled pregnant plasma contained oestrogens, among which the presence of oestriol was satisfactorily demonstrated by paper chromatography, thin layer chromatography, methylation and acid fluorescence during isotope dilution studies.