Two previable human foetuses were perfused at midpregnancy with corticosterone-1,2-3H and the metabolites formed were isolated from the various tissues and perfusates. With the exception of the adrenals, the bulk of the radioactive material present in the perfusates and various foetal tissues was in an unconjugated form. The following compounds were isolated in a radiochemically homogeneous form: 21-hydroxy-pregn-4-ene-3,11,20-trione (liver, intestines, lungs, adrenals, residual foetal tissues and perfusates), 11β,20β,21-trihydroxy-pregn-4-en-3-one (liver, intestines, lungs, residual foetal tissues and perfusates), 3α,11β,21-trihydorxy-5β-pregnan-20-one (liver, intestines, lungs, residual foetal tissues and perfusates), 6β,11β,21-trihydroxy-pregn-4-ene-3,20-dione (liver, intestines, lungs, residual foetal tissues and perfusates), and 11β-hydroxy-pregn-4-ene-3,20-dion-21-yl sulphate (liver, intestines, lungs, adrenals, residual foetal tissues and perfusates). In addition, 20β,21-dihydroxy-pregn-4-ene-3,11-dione (liver, lungs, residual foetal tissues and perfusates) was identified by its chromatographic behaviour prior and subsequent to the formation of various derivatives. Finally, small quantities of corticosterone, 11β,20β,21-trihydroxy-pregn-4-en-3-one and 3α,1 1β,21-trihydroxy-5β-pregn-20-one were detected in the glucuronide fraction of the intestines, residual foetal tissues and perfusates. A concept is presented describing the metabolism of corticosterone in the human foetus at midpregnancy.