In transpiration cooling of various structural elements in gas turbines, the coolant has to be ducted within passages to the porous walls through which it is ejected into the gas stream. The passages, often arranged in rotating parts, have to be designed in such a way as to ensure the proper local distribution of the coolant. In this report, a method is presented by which either the local permeability can be predicted. The method is based on a one-dimensional treatment of the gas flow through a rotating channel with varying cross section and partially porous walls. The inlet pressure into the channel and the outside pressure along it are assumed prescribed. It is also stipulated that the passage ends blindly. However, the method can easily be extended to cover the situation where a certain mass flow leaves the open end of the passage.