We have developed a model for combined morphological and functional in vitro studies of the isolated mediobasal hypothalamus (MBH) by considering two prerequisites: (1) the tissue must be well preserved, free of morphological artefacts and functionally unimpaired until the end of the in vitro incubation, and (2) the tissue must be processed for morphology in optimal conditions. To test our model we have studied some aspects of the luteinizing hormone-releasing hormone (LHRH) system in 4-month-old male Sprague-Dawley rats. After decapitation the MBH was isolated and put in a flask containing 0.5 ml Hepes-buffered Locke’s medium gassed by 5 ml/min of O2/CO2 (95%/5%) and shaken in a water bath at 37 °C. After a 10-min washing, the medium was changed twice at an interval of 20 min. After the in vitro incubation the tissue was satisfactorily preserved as judged by light- and electron-microscopic analysis. LHRH, somatostatin and thyrotropin-releasing hormone could be demonstrated by alkaline phosphatase or peroxidase-antiperoxidase im-munohistochemistry on semithin sections and by immunogold technique on thin sections. The LHRH secretion was close to basal values after 30 min of incubation (22.1 ± 4.8 pg/MBH) and then remained constant for another period of 20 min (17.6 ± 2.6 pg/MBH). During the second 20 min of incubation LHRH secretion increased in presence of 61.6 mM K+ (110.7 ± 8.7 pg/MBH). Thus the isolated hypothalamus was excitable until the end of the in vitro incubation. We conclude that this model can be successfully used for combined morphological and functional st