Oxytocin secretion in dairy cows was measured during milking by means of a specific radioimmunoassay. Basal oxytocin concentrations before milking recorded from 147 milkings from 21 cows, where an assay method with enhanced sensitivity was employed, were 1.5 +/- 0.6 pmol/l and there was no evidence for any conditioned release of oxytocin. A 1-min manual stimulation before milking evoked a variable but distinct increase in oxytocin concentrations in 188 out of 195 milkings performed using 29 cows. Despite the high variation in absolute concentrations five types of typical secretion pattern could be distinguished. There was no obvious relationship between patterns or absolute concentrations of oxytocin and milk-flow characteristics. Evidence is given that milk ejection seems to follow the threshold principle in that small releases of oxytocin up to a range of 3-5 pmol/l plasma are sufficient to evoke maximum milk ejection.

Milking cows with a 1-min manual stimulation (treatment 1) and without any udder preparation (treatment 2) was compared by application of an improved, highly sensitive radioimmunoassay for oxytocin and recordings of milk-flow curves. Both treatments caused the release of oxytocin, but treatment 2 generally seemed to be less efficient. Milking characteristics supported the advantage of manual stimulation; milk yield and milk flow were significantly higher, while 'machine-on' time was shorter. This clearly indicates the importance of the right timing of release of oxytocin before commencement of milking. Substitution of stimulation by an i.v. injection of 0.5 i.u. oxytocin (treatment 3) resulted in milking parameters very similar to those of treatment 2. This implies that manual stimulation has other effects besides the secretion of oxytocin which are also responsible for optimal milk removal.

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