A.R.C. Institute of Animal Physiology, Babraham, Cambridge, CB2 4AT (Received 19 January 1978) Vasopressin is present in nerve terminals of the median eminence (e.g. Silverman & Zimmerman, 1975) and the hormone may possess corticotrophin releasing activity (Pearlmutter, Rapino & Saffran, 1974, 1975). Thus, in experiments designed to investigate the release of peptides from nerve terminals of the median eminence by application of electric stimuli to synaptosomes prepared from mediobasal hypothalamic tissue, we also hoped to establish that vasopressin was secreted from these nerve-endings. Synaptosomes were prepared from tissue obtained from groups of 40 female rats (Gray & Whittaker, 1962) and incubated for 40 min in aerated Locke solution containing 2 mm-bacitracin to minimize peptidase activity. The incubation chamber consisted of an oxygen electrode (Rank Bros, Bottisham, Cambridge) modified to enclose the incubation medium between two silver electrodes. Pulsed stimuli were delivered between these electrodes (5–50 Hz; 1 ms duration; monopolar;