1. The intracellular Ca2+ concentration ([Ca2+]1) was monitored in single magnocellular neurones freshly isolated from rat supraoptic nucleus. Application of 100 nM vasopressin increased [Ca2+]1. Two types of [Ca2+]1 responses were observed: (i) a transient response, displayed by 86% of the vasopressin-sensitive neurones, and (ii) a sustained response displayed by 14% of the vasopressin-sensitive neurones. 2. Among responding neurones, 52% were vasopressin sensitive, 44% were oxytocin sensitive and 4% were sensitive to both peptides. 3. Responses to vasopressin were dose dependent, showed a progressive desensitization after successive applications, were specifically blocked by the V1a vasopressin receptor antagonist, SR 49059, and were unaffected by the oxytocin receptor antagonist, d(CH2)5OVT. 4. Vasopressin responses were completely suppressed by the removal of external Ca2+. 5. The intracellular Ca2+ mobilizers, caffeine and tBuBHQ, did not affect resting or vasopressin-induced [Ca2+]1 changes. Thapsigargin (200 nM) on its own evoked an increase in [Ca2+]1, and reduced the [Ca2+]1 increase evoked by vasopressin by 52%, suggesting that thapsigargin-sensitive Ca2+ stores are partially involved in the vasopressin response. 6. Immunocytochemical identification revealed that vasopressin-responding neurones synthesize vasopressin whereas oxytocin-responding neurones synthesize oxytocin. 7. In conclusion, vasopressin- (partially external Ca2+ dependent) and oxytocin (totally external Ca2+ independent)-induced [Ca2+]1 changes are mediated by specific receptors. In addition, vasopressin and oxytocin neurones are specifically autoregulated by their own peptides.