The (spot) noise figure of a two-port is regarded as a parameter of a network model which represents the two-port when the input termination is specified. Noise figure and effective temperature are calculated by replacing noise sources at one terminal pair with equivalent sources at another port. Available (or exchangeable) power gain appears implicitly when power spectra of equivalent sources are calculated. Special provisions for negative input and output conductances are therefore unnecessary. Extensions to linear three-port models are discussed. © 1962.