To facilitate future development of the cooled gas‐turbine, more test information is needed on the effectiveness of cooling in an actual turbine operating at high gas‐temperatures. Part I of this paper deals with some design aspects of a single‐stage experimental turbine built to enable an experimental investigation to be carried out on the air cooling of nozzles and blades. The turbine, built for operation at high gas‐temperatures, was fitted with internally air‐cooled blades having a large number of small cooling passages running the whole length of the blades. A description is given of the pressed powder technique used to introduce the small passages in blocks of heat‐resisting material from which the blades could be machined. Mention is made of some of the difficulties encountered in this method of manufacture and also of the need for careful consideration of suitable methods of disposal of the cooling air when internally cooled nozzles and blades of this form are used.