A 24-h rhythm of plasma PRL is present in fetal sheep. This rhythm
is synchronized to an environmental clue (zeitgeber). We determined
whether the light-dark cycle (L:D) is a zeitgeber for the fetal PRL
rhythm and, if so, whether the mother might convey this zeitgeber to
the fetus. We kept nine ewes (twin pregnancies) in constant light (L:L)
and five ewes (singleton) in 14:lO L:D from 110 days gestation. Fetuses
and mothers were catheterized at 119 days gestation. Blood
samples were taken hourly for 24 h after 16 days under L:L or L:D.
A mean 24-h rhythm of PRL was found (by RIA) in fetuses under L:D,
but not in those under L:L. However, fetuses under L:L showed individual
24-h PRL rhythms (cosinor analysis) whose acrophases were
distributed around the clock. Nonsynchronized rhythms persisted
after 23 and 30 days of L:L. Acrophases of PRL rhythms within a set
of twins were closer than those between sets, suggesting that twins
were responding to a common signal. These findings indicate that the
L:D cycle is a zeitgeber for the PRL rhythm in fetal sheep and suggest
that the mother might convey the zeitgeber.