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An investigation has been carried out in goats of the possible seasonal factors which might influence the autumn decline in the average concentration of prolactin released at milking. Serial blood samples were taken during milking, once a month between August and December 1972, from groups of mated, non-mated and ovariectomized goats and also from goats maintained in summer light conditions (17 h light: 7 h darkness). Since high levels of progesterone occur in the circulation during the major part of the oestrous cycle and pregnancy in the goat, the same sampling procedure was carried out earlier in the year (June–August) in animals which had been implanted with progesterone tablets.

Although high levels of progesterone in the circulation inhibited the release of prolactin at milking, oestrus, mating and pregnancy were not found to be responsible for the fall in prolactin levels in the blood in the autumn. In each of these cases the concentration of prolactin released at milking decreased uniformly between August and October. Housing goats in summer lighting conditions, however, maintained the release of prolactin at milking at a consistently high level from August until December, but had no effect on the milk yield which declined in a manner similar to that of the control animals. It is concluded that daylength is the predominant factor governing the autumn decline in the concentration of prolactin released at milking and that the decline is not the major cause of the fall in milk production during late lactation in goats.