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Blood oxytocin was assayed on the lactating guinea-pig mammary gland.

Transient release of oxytocin was detected in the range of 9–889 µu./ml. plasma in the external jugular vein blood of cows during machine milking. In 24 experiments oxytocin was detected in 17; in nine of these, hormone release by conditional reflex was obtained in response to auditory or visual stimuli associated with the milking routine. All stimuli associated with the milking routine caused oxytocin release. There was no difference in the plasma concentrations detected in response to conditional and nonconditional stimuli. In this and previous work no release of oxytocin was detected in 32% of experimental machine milkings although in most cases milk yields were normal.

Milk ejection was accompanied by increases in intramammary pressure from 12·5–24 mm. to 23–44 mm. Hg; the pressure after ejection was maintained virtually constant for over 1 hr. There was no initial drop in pressure, associated with a neural segmental reflex. Rise in intramammary pressure accompanied the detection of oxytocin in jugular blood in four out of seven experiments. The pressure changes and latency could be simulated by rapid intravenous injection of 100 m-u. synthetic oxytocin. The half-life of synthetic oxytocin in the circulation was 127 ± 9·3 sec. (range 85–164 sec.).