Evoked potential measurements are influenced by several factors that are present even in the absence of the stimulus under study. Such "noise" is generated mainly by recording equipment, by stray electromagnetic fields and by any ongoing electrical activity that is unrelated to stimulus presentation. Spectral studies of the EEG have shown, however, that ongoing activity cannot be described as white noise. An alternative method is developed for the case where the white noise requirement is not met. We assume that the noise signal is a sample from a second order stationary process. This assumption leads to a procedure that we have called "Multivariate Analysis with Stationary Error" (MASE).