The star-winding resistances of a 132-kV power transformer were measured by superimposing direct current upon the 50–c/s load current while the transformer remained in service. From these measurements the winding temperature could be derived to within 3°C—an uncertainty due chiefly to the occurrence of the small fluctuations in system voltage which are inseparable from a normal power system in service. The special measures necessary to achieve this degree of accuracy include the provision of a filter stage with electronically enhanced time-constant. In comparison with the measurement of winding resistance of a transformer in service, results by a similar method applied to a power transformer while undergoing a short-circuit heat run show that winding temperatures could be assessed within 0.5°C. By measuring in this way the changes in temperature which follow a sudden change in load, close agreement has been found with the cooling correction rule given in B.S. 171: 1936. An Appendix discusses variations in direct-current distribution between the three star windings.