Acid hydrolysis is widely used in the estimation of urinary oestrogens. Its main advantages over enzymic hydrolysis are its speed and simplicity. Its main disadvantage is a loss of 10–20% of oestrone and oestriol, depending on the dilution of the urine. The losses of oestradiol and less stable oestrogen metabolites may be much greater (Brown, 1965). The conditions for acid hydrolysis must be chosen to produce complete hydrolysis of conjugates with minimal destruction of oestrogens. Boiling under reflux for 1 hr. with 15 vol. of conc. HCl/100 vol. urine has been found to be a satisfactory compromise and has been widely adopted. In laboratory practice this step is inconvenient as well as slow. In routine assays boiling large numbers of urine samples under reflux is dangerous, gives rise to unpleasant smells and uses a good deal of valuable bench space. Hydrolysis under pressure was described by Frandsen (1963) as a