Concentrations of prolactin in serial blood samples from goats before, during and after milking throughout lactation

Two experiments are described in which prolactin was measured in (a) serial blood samples taken at monthly intervals (April-November) from six lactating goats before, during and after milking and (b) in similar blood samples taken on days 2, 7 and 12 after parturition. The quantity of prolactin released at milking (April-August) varied both between goats and within the individual goat. No correlation was found between the duration of milking and the quantity of prolactin released into the circulation (r equals minus 0-24, P greater than 0-1) or the average concentration of prolactin found at milking (April-August) and the average daily milk yield of the individual goat (r equals minus 0-57, P greater than 0-1). No evidence was found for a release of prolactin conditioned by events normally preceding milking. The results showed a correlation between the average concentration of prolactin released at milking each month and the average milk yield of the six goats (r equals 0-67, P smaller than 0-05). The physiological significance of this correlation is discussed. No correlation was found between the average concentration of prolactin released during early lactation and the average milk yield of the goats (r equals 0-36, P greater than 0-1).