Incubation of labelled pregnenolone with adrenal slices from two midgestation foetuses resulted in the formation of 21-hydroxypregnenolone, but no progesterone was detected. Incubation of labelled 21-hydroxypregnenolone yielded both deoxycorticosterone and corticosterone. When two midgestation foetuses were perfused with labelled 21-hydroxypregnenolone, deoxycorticosterone and corticosterone were isolated from their adrenals. In situ perfusion of two midgestation placentas at laparotomy with 21-hydroxypregnenolone yielded large quantities of deoxycorticosterone both in the placentas and perfusates. Following a single cycle perfusion more than 50% of the radioactive material recovered from the placentas and more than 10% of that recovered from the perfusates was in the form of deoxycorticosterone. A scheme is presented describing the »Δ5 pathway« of corticosteroid biosynthesis in the adrenals of perfused previable foetuses at midgestation.