In this study EEG-spectral parameters like mean alpha-power values and center frequency values are computed as maps and compared using significant probability mapping. The parameters are used to describe changes of the functional state of the brain after neuroleptic depot injection (Haldol-Decanoate) in schizophrenic patients. The absolute alpha-power increased significantly ( p<0.01), especially within the first week after injection mostly in the left hemispheric regions. The center frequency decreased most significantly ( p<0.02) between the first and second week after haldol depot injection at the right occipital regions. Both parameters are efficient to describe functional changes correlated with clinical symptomatology, even 3–5 days earlier. Thus, EEG power and center frequency mapping can be used very efficiently to estimate the optimal time between sequential neuroleptic depot injections.