The aim of the present study was to examine suckling-related plasma levels of oxytocin and prolactin in early and established lactation and to correlate hormone profiles to success of lactation performance. Fifty-five primiparous women participated in the study. From each, 18 blood samples were drawn in connection with breast-feeding on day 4 post partum and after 3-4 months. Oxytocin and prolactin levels were determined with radio-immunoassay. Basal levels of both hormones were significantly higher 4 days post partum than 3-4 months later and after weaning. Basal prolactin levels fell significantly within 24 h of weaning. Oxytocin and prolactin levels rose in response to breast-feeding–an effect which persisted during the lactation period. The suckling-induced release of prolactin–but not that of oxytocin–was related to basal hormone levels. Basal as well as stimulated oxytocin levels obtained 4 days and 3-4 months post partum correlated significantly, indicating that each woman has an individual, characteristic level of this hormone. Milk yield did not correlate with oxytocin or prolactin levels, but prolactin levels recorded 3-4 months post partum did correlate with the remaining period of breast-feeding. In addition, mothers who breast-fed exclusively 3-4 months post partum had significantly higher oxytocin and prolactin levels than those who gave supplementary feed. There was a significant correlation between oxytocin levels at 4 days and birth weight of the infant.