OBJECTIVE There have been anecdotal reports of increased maternal thirst during breast feeding, but the physiological mechanisms remain obscure. We have assessed and quantified the stimulation of maternal thirst during breast feeding (suckling), and correlated the changes with plasma oxytocin and vasopressin levels. METHODS A within subject design was used with each subject acting as her own control. Each subject participated in a suckling period and a control non-suckling period the order of which was counterbalanced. METHODS Ten healthy breastfeeding women ranging from 28 to 52 days post partum. METHODS Thirst (assessed by a visual analogue scale), plasma vasopressin, plasma oxytocin (pOT), plasma osmolality and haematocrit, blood pressure, volume of milk transferred to baby, volume of water drunk immediately following each experimental period. RESULTS Thirst increased significantly more over a suckling period than over a comparable control period (P = 0.013). The peak level of thirst correlated with the volume of water consumed to satiety following the suckling period (r = 0.7, P = 0.024). As expected, pOT levels rose significantly in response to suckling (P < 0.001). Six women demonstrated a close relation between thirst and oxytocin response during the suckling period. Despite significant changes in thirst during suckling there was no increase in plasma vasopressin or in osmoregulatory parameters. CONCLUSIONS This study confirms that suckling is a potent stimulus to thirst in the mother, and is not associated with vasopressin release or dependent on any measurable alterations in osmoregulation. What actually stimulates thirst during breast feeding remains unknown, but there are two potential explanations for these observations: (1) suckling sends nerve impulses to the paraventricular and supraoptic nuclei in the hypothalamus which may have afferents within the central nervous system which stimulates a thirst response simultaneous with oxytocin release; (ii) a learned anticipation of thirst may be occurring in a situation associated with expectant fluid loss to preserve homeostasis.