Relation between daylight ratio, plasma progesterone levels and timing of nidation in mink (Mustela vison).

Mink were mated between 17 February and 22 March. In females kept in natural daylight concentrations of progesterone, measured by radioimmunoassay, began to rise between 25 and 30 March, whatever the date of mating. After reaching peak values of 40--160 ng/ml, progesterone concentrations decreased before the end of pregnancy. In females given 14 h light/24 h immediately after mating, the rise of progesterone began a few days earlier, indicating that the extra light induces earlier progesterone secretion, nidation and parturition.