Negative feedback, in which a hormone directly or indirectly restricts its own release, provides the servo-control observed in many endocrine situations. Such control could apply to oxytocin, for many studies claim that exogenous oxytocin reduces milk production (and ejection) in rats and other species (Carrol, Jacobsen, Kassouny, Smith & Armstrong, 1969; Mena & Beyer, 1969; Kuhn & McCann, 1970; Deis, 1971). In these studies oxytocin has always been given in amounts or in a form which we must now regard as somewhat unphysiological. One milliunit is a physiological dose for a rat and during the prolonged nursing of the rat pulses of this size are released at regular intervals of 5–15 min (Wakerley & Lincoln, 1971; Lincoln, Hill & Wakerley, 1973).
Experiments were conducted to determine whether oxytocin, in low doses, would inhibit or re-modulate the pulsatile release of endogenous oxytocin during suckling. Rats were taken at day 9–10 of