The Lowicryl K4M low-temperature embedding technique was modified for brain tissue to facilitate antigen localization in neuronal structures by using postembedding immunoelectron microscopy. Rat brains fixed by perfusion with a mixture of 1% glutaraldehyde and 1% paraformaldehyde, were sectioned into 200-microns-thick coronal sections, and the appropriate area of the sections further microdissected into small samples (200 x 700 x 700 microns). These samples were processed for K4M embedding using prolonged incubation times at each step. This resulted in a homogeneously infiltrated tissue with good preservation of ultrastructure and antigen reactivity. Despite the lack of osmium fixation, the morphology of the synapses was clearly identifiable.