Summary. One sow bled at 30\p=n-\60-minintervals for 48 h at 5 and 4 days before parturition had mean \m=+-\ s.e.m. relaxin levels of 5\m=.\0\ m=+-\0\m=.\48ng/ml and 5\m=.\5\ m=+-\0\m=.\44 ng/ml for each 24-h period respectively. This sow and another were bled at frequent intervals during parturition; both showed considerable fluctuations in their relaxin levels but no consistent peaks in relation to each birth. Mean levels during parturition were 10\m=.\7\ m=+-\ 0\m=.\46 ng/ml and 13\m=.\4\ m=+-\ 0\m=.\81ng/ml respectively, both significantly higher than the levels at 4 and 5 days before birth.Relaxin levels in two lactating sows rose acutely during nursing, showing a 3-fold rise in one animal and an 8-fold rise in the other. Results from a third sow during an extended period of nuzzling and sucking by the piglets showed multiple peaks of relaxin immunoactivity associated with each nuzzling/sucking stimulus.
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