The nature of the circadian signal from the suprachiasmatic nucleus (SCN) required for prolactin surges is unknown. Because the SCN neuronal circadian rhythm is determined by a feedback loop of Period 1, Period 2, and Clock gene expressions, we investigated the effect of SCN rhythmicity on prolactin surges by disrupting this loop. Because lesion of the locus coeruleus (LC) abolishes prolactin surges and these neurons receive SCN projections, we investigated the role of SCN rhythmicity in the LC neuronal circadian rhythm, as a possible component of the circadian mechanism regulating prolactin surges. Cycling rats on proestrous day and estradiol-treated ovariectomized rats received injections of antisense or random-sequence deoxyoligonucleotide cocktails for clock genes (period 1, period 2, and clock) in the SCN, and blood samples were taken for prolactin measurements. The percentage of tyrosine hydroxylase-positive neurons immunoreactive to FOS-related antigens (FRA) was determined in ovariectomized rats in 12:12 h light:dark (LD) or constant dark (DD) submitted earlier to the cocktail injections. The antisense cocktail abolished both the proestrous and the estradiol-induced prolactin surges observed in the afternoon and the increase of FRA expression in the LC neurons at Zeitgeber 14 in LD and at circadian time 14 in DD. Because SCN afferents and efferents were probably preserved, the SCN rhythmicity is essential for the magnitude of daily PRL surges in female rats, as well as for LC neuronal circadian rhythm. SCN neurons therefore determine PRL secretory surges possibly by modulating LC circadian neuronal activity.