Substance P (SP)-, neurokinin A (NKA)-, neurotensin (NT)-, neuropeptide Y (NPY)- and vasoactive intestinal polypeptide (VIP)-like immunoreactivity (Ll) were measured and characterized by specific radioimmunoassays (RIA) and reverse phase high performance liquid chromatography (HPLC) in extracts of rat brain. Concentrations of SP-Ll, NKA-Ll and NT-Ll in brains of decapitated animals were 59, 49 and 64 percent lower compared to those found in animals sacrificed by focused microwave irradiation (MW). In contrast, no difference in brain NPY-Ll and VIP-Ll levels was found between animals killed by MW and decapitation. HPLC chromatograms of SP-, NKA-, NT- and NPY-Ll showed the same immunoreactive components in extracts of brains from both groups of animals. Thus, no additional immunoreactive components were formed by MW compared to those found after decapitation. The present findings may reflect an MW-induced inhibition of peptidase activity or, perhaps, a more efficient extraction of certain neuropeptides following MW treatment. The results imply that the traditional methods of sacrifice may result in the measurement of spuriously low tissue concentrations of some peptides, e.g. tachykinins, in brain.