Statistical information on the relationship between amplitude and frequency of occurrence of short duration transients in low-voltage supply mains has been obtained using multi-channel recording equipment. The equipment responds to pulses having durations exceeding 0-3 JJLS and amplitudes greater than certain predetermined levels. Either 4 or 6 level recorders have been installed for about 3 months at each of 19 sites chosen from industrial and domestic premises, and Area Board sub-stations. Analysis of the 36 000 transients which have been recorded show that the frequency of occurrence decreases approximately by 10 to 1 for a 2 to 1 increase in amplitude. Transients of up to 50 V peak occur frequently but one of 500 V or more is rare and is unlikely to occur at or near to sub-station transformers.