Peripheral concentrations of melatonin in the neonatal mammal are low and the diurnal rhythm is not apparent
during the first days or weeks of life. Maternal milk provides a rhythmic although low source of melatonin to the
newborn. Diurnal rhythm of serum melatonin was monitored in Friesian calves from birth to 4 weeks of age. Some
of the calves under 1 week of age had no clear nocturnal elevation of serum concentrations of melatonin. Thereafter
the nocturnal melatonin elevation was apparent in all calves and of the same amplitude as in adult cattle. The effect
of oral intake of night-time milk or milk supplemented with low doses of synthetic melatonin was studied during
day-time in a group of Ayrshire calves aged 1 to 5 weeks. Two and a half or 3 litres of night-time milk with a mean
melatonin concentration of 40 pg/ml, providing 0·1 to 0·12
μg melatonin, did not cause any change in the day-time
serum concentrations of melatonin, neither did addition of melatonin in doses up to 10
μg per animal. Only the
highest dose of melatonin 200
μg in 2 l of milk (0·1
μg/ml) caused an elevation of day-time serum concentrations of
melatonin. These studies show that the pineal gland of a very youg calf is functional and that the concentration of
melatonin in blood does not reflect the concentration in milk at the time of feeding. The presence of melatonin in
milk suggests, however, its potential significance for the neonate.