Since the effects of injecting oxytocin into ewes just after artificial insemination were to be tested, the experiment described was designed to determine a range of doses of synthetic oxytocin which would produce 'milk let-down' in ewes. Intramuscular injection was chosen as the most practical route of administration since it was planned to use a large number of sheep in the subsequent experiments.
Martinet & Denamur (1960) have shown that both massage of the udder and i.v. injection of 10–50 m-u. oxytocin cause similar increases in intramammary pressure in goats and sheep and Debackere & Peeters (1960) and Debackere, Peeters & Tuyttens (1961) have shown that intramammary pressure in ewes was increased by distension of the vagina; a similar response was obtained by i.v. injection of 20–50 m-u. oxytocin. Since Fitzpatrick (1960) had found that oxytocin injected i.v. was 2·25 and 2·5 times more potent than when injected i.m., the