A research programme has been commenced which is designed to determine the mechanisms of electric breakdown in insulating liquids under various conditions. The paper describes experimental equipment and techniques, and some results which have been obtained on transformer oil.Special equipment includes a transparent pressure chamber with an internal glass test-vessel, which enables the liquid under test to be subjected to hydrostatic pressures up to 500 lb/in2and voltages up to 100 kV peak. A protective circuit hasbeen developed, incorporating triggered spark-gap, which limits the duration of breakdown currents to a few microseconds.A wide range of breakdown strengths has been obtained, varying from 5 000 kVpeak/cm with 1-microsec impulses, to below 100 kV(r.m.s.)/cm for a fibre contaminated oil under a.c. test conditions. The removal of fibres raises the a.c. strength of otherwise untreated oil to 400 kV(r.m.s.)/cm.It is concluded that a breakdown mechanism initiated by cavitation is operative in certain circumstances.