The opening of plain aluminium diaphragms, which were pressurized to their natural rupture pressures, was recorded photographically (using a sub-microsecond flash) and by a photoelectric method; there was good agreement between these two methods when the time dependence of the aperture size was considered. Photographic observations were also made of the piercer-assisted bursting of cruciformed copper diaphragms which were pressurized to at least 90% of their natural rupture pressure. In both methods of bursting diaphragms the initial splitting along lines of natural or enforced weakness (producing a small aperture) occupies a relatively long time compared with that taken for the petals so formed to fold back fully.