Diurnal variations of plasma gonadotrophins in male domestic ducks during the sexual cycle

Blood samples were collected every 3 hours for at least 24 hr from a group of 12 immature, male domestic ducks during their gonadal growth phase and at the height of the breeding season. The samples were assayed for FSH and LH. No clear pattern of diurnal variation could be defined for either hormone at any time of the sexual cycle. Rapid changes in hormone levels were, however, observed, but no relationship could be established between the variations in different males at the same time of the year or between variations in the same male at two different seasons. Individual differences in plasma gonadotrophins are remarkably stable during a single day and even over several months, particularly for LH.