In order to investigate the synthesis of squalene and lanosterol in the human foeto-placental unit at midgestation, two complete foeto-placental units, two isolated midgestation foetuses and two isolated midgestation placentas were perfused with [14C] sodium acetate immediately following their removal at laparotomy. The perfusions were carried out at 36°C for 90 minutes, administering 5.0 mCi (foeto-placental units), or 2.5 mCi (separate perfusions of foetuses or placentas) amounts of [14C] sodium acetate. Chromatographic evidence indicated the presence of [14C] squalene-like material in the extracts of all adrenals and livers. Radiochemically homogeneous squalene was isolated in the form of squalene dodecabromide from one adrenal and two liver extracts, which were studied more in detail. In the extracts of all placentas, testicles, carcasses and perfusates, the presence of any [14C] squalene was excluded by crystallization. Sufficient material for the identification of [14C] lanosterol was present in the extracts of three adrenals, three livers, four testicles and two carcasses. From all these extracts radiochemically homogeneous [14C]-lanosterol was isolated. The [14C] lanosterol-like material present in all perfusates was so limited that it precluded further identification. In the extracts of three placentas, the presence of any [14C] lanosterol was excluded by crystallization. From the extract of a separately perfused placenta a minute amount of radiochemically homogeneous [14C] lanosterol was isolated. It is concluded that the midgestation human foetus is capable of synthesizing major quantities of squalene and lanosterol. Little, if any, squalene and lanosterol are formed by the midgestation human placenta.