Recordings of intramammary pressure have been used in recent experiments to demonstrate the intermittent pattern of milk ejection (ME) in the rat (Wakerley & Lincoln, 1971; Lincoln, Hill & Wakerley, 1973). This technique provides 'on-line' but indirect evidence of oxytocin release. Other naturally occurring substances (bradykinin, 5-hydroxytryptamine, acetylcholine and vasopressin) can initiate contractions of the mammary gland and thus mimic the effect of oxytocin (Bisset, Clark, Haldar, Harris, Lewis & Rocha e Silva, 1967). The following experiments were designed to identify oxytocin as the active milk-ejecting principle released during suckling. Bisset, Clark & Haldar (1970) described an in-vivo method of identifying oxytocin using an antagonist, N-carbamyl-O-methyl-oxytocin, which acts as a competitor for oxytocin receptors. The effects of this antagonist on the contractions of the mammary gland during natural ME were examined. Lactating rats were anaesthetized with tribromoethanol (Lincoln et al. 1973) and two mammary glands in the