This paper describes a laboratory on investigation of the longitudinal insulation withstand of HV circuit-breakers under the joint effect of pollution and electric stresses representative of those found in service. For this purpose, a simplified laboratory representation of the actual electrical stresses was studied. The stresses included, in addition to the continuous operating voltage, temporary overvoltages deriving from synchronization operations and from opening capacitive loads. Several types of circuit-breakers with rated voltages ranging from 170 kV to 420 kV were tested. As regards the artificial contamination of the surface insulation of the circuit-breakers, both salt fog and solid layer methods were used. The results proved that the behaviour of the longitudinal insulation of circuit-breakers can be critical in service. Therefore, when designing this type of equipment, adequate creepage distances and profiles for the hollow insulators must be provided. In the most severe cases, special protective systems such as dead tanks, and proper maintenance practices such as cleaning and greasing should be considered.