Through the combined demonstration of retrograde transport of horseradish peroxidase (HRP) and immuno-cytochemical staining of thyrotropin-releasing hormone (TRH), the TRH neurons that project to the lateral septum were identified. Following the injection of HRP into the lateral septum, retrogradely labelled neurons were detected in the bed nucleus of stria terminalis (BNST), the ventral pallidum, the anterior commissural nucleus, the lateral preoptic nucleus and the perifornical region. Some neurons in the BNST and the perifornical region were found to contain both TRH and HRP. In contrast, no TRH-containing neurons were labelled with HRP in the paraventricular nucleus, the putative thyrotropic area. These data suggest that TRH neurons projecting to the lateral septum were different from those which send fibers to the median eminence.