Abstract
Effects of photoperiods on prolactin (PRL) secretion were investigated by keeping female Saanen goats under artificial long days (16L8D) or short days (8L16D). Exposure to experimental photoperiods was started in December 1980 (day 0) and continued for about a year. Plasma PRL of the long-day group was elevated to the levels much higher than those of the short-day group. Effects of photoperiods on the diurnal fluctuation of plasma PRL were also evident on selected days when blood samples were taken at short intervals (day 24 and 118): being enhanced under long days and damped under short days. In July and August 1981, a marked elevation of PRL levels was noted in both groups, probably due to high ambient tem-perature which might have masked the effects of short-days. In the long-day group, goats were anovulatory when PRL levels were high (day 78-148), however, by day 200 they re-sumed estrous cyclicity spontaneously in association with the decrease in plasma PRL, sug-gesting that there might be an intrinsic mechanism which gradually increases the critical day lengths.
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