The present work investigates the possible existence of molecular heterogeneity of endogenous neuropeptide Y (NPY) in the rat brain and, if existing, whether it is affected by repeated electroconvulsive stimuli (ECS). Different column chromatographic techniques (gel-permeation chromatography, HPLC, ion-pair reverse-phase HPLC) were used combined with immunochemical methods based on antisera directed towards two different epitopes of NPY (antiserum N1, directed towards the midportion of NPY, and antiserum C1, directed towards the C-terminal end of NPY). Following repeated ECS increased concentrations of NPY were seen in the occipital cortex and hippocampus (P < 0.001) when analyzed by direct radioimmunoassays (RIA). Chromatographic characterization showed that the NPY mainly consisted of intact NPY (1-36) and sulphoxidated form of NPY, no short C-terminal homologues were found. This is of importance since it has been shown that NPY (1-36) has different biological properties compared to C-terminal homologues.