Prolactin in the ram: seasonal variations in the concentration of blood plasma from birth until three years old.

Peripheral prolactin levels were measured by radioimmunoassay in nine Ile de France rams, born in autumn and reared under natural conditions of light and temperature until three years old. During December, when the rams were 80-100 days old, a brief and transitory peak of prolactin occurred (70 ng PS6/ml plasma +/- 10 ng). During each following year, the prolactin level fell in winter to approximately 20 ng/ml. However, in summer the prolactin was 4 times higher than the winter level during the first year and 11 times higher during the 2nd and 3rd years. A possible role of prolactin variation throughout the breeding season is discussed.