[Seasonal variation in the nyctohemeral rhythm of prolactin in cows].

Seasonal variation of prolactin nyctohemeral rhythm has been studied by radioimmunoassay on female cattle of the white blue belgian breed. Animals are divided into three groups as age is concerned : 6 calves, 6 heifers and 5 lactating cows. In december all groups of animals show a significant increase of plasma prolactin concentration between 5 a.m. and 7 a.m. Calves and heifers have a second secretion period either at the end of the afternoon between 17 and 19 p.m. or during the evening between 20 and 23 p.m. The circadian rhythm observed in may with heifers is similar to that in december with two waves whose relative importance is different at the two seasons considered. Nyctohemeral rhythm of prolactin in cattle is age and season dependent.