Prolactin response to suckling was studied in a group of fully breast feeding women (N = 58) between 4-6 weeks postpartum. Basal, suckling stimulated and the increment of prolactin showed wide individual variations. Basal prolactin concentrations varied from 140 to 4,600 mIU/l, suckling stimulated prolactin from 400 to 5,600 mIU/l and the increment of prolactin from 40 to 4,160 mIU/l. Basal (p = 0.0395) and suckling stimulated (p = 0.0423) prolactin concentrations significantly increased as the number of night breast feeds increased and the suckling stimulated (p = 0.0218) prolactin concentrations significantly increased as the number of breast feeds/24 h increased. However, the magnitude of the rise in prolactin in response to suckling was not dependent on basal prolactin concentration. Basal, suckling stimulated or the increment of prolactin were not significantly different between subjects having different breast feeding frequencies, when the subjects were grouped according to the number of breast feeds. These differences may be due to the large individual variation in prolactin concentrations seen in women having similar breast feeding frequencies which may arise from individual variations in hypothalamic--pituitary response to suckling.