Using radioimmunoassay for total immunoreactive blood neurophysins (IRN) a significant increase 30 min after the onset of suckling was shown in 26 normal lactating women. On the 1st day postpartum a constant increase is only observed in 12 patients while a significant increase is clearly shown for all patients from the 2nd to the 6th day. These variations are similar to those observed for prolactin blood levels; there is no relation between basal, suckling stimulated, or ergocryptine inhibited prolactin blood level on the one hand and IRN blood level on the other hand.