1. Dehydroepiandrosterone sulphate was hydrolysed by human placental steroid 3beta-sulphatase in H(2) (18)O. Equimolar amounts of dehydroepiandrosterone and sodium sulphate were similarly incubated as controls. After incubation, unconjugated steroid was extracted with ether and sulphate precipitated as barium sulphate. Both were analysed for (18)O-content by mass spectroscopy, the sulphate as carbon dioxide after initial pyrolysis with graphite. 2. In duplicate experiments, amounts of (18)O equivalent to 67% and 72% respectively of the theoretical content calculated for rupture of the O-S bond were present in the sulphate. As the enzyme preparation used was a microsomal preparation containing unenriched endogenous sulphate and phosphate, and as no incorporation of isotope was found in the steroid, it is concluded that the placental enzyme effects hydrolysis by rupture of the O-S bond.