1 Plasma kininogen concentration of the female rat did not change during the oestrous cycle but increased two-fold during gestation. Plasma kininogen did not change during parturition, rose in the first puerperal day and then rapidly declined to non-pregnant levels.2 Free kinin levels in the blood of non-pregnant female rats were low and inconstant. Free kinin was undetectable in the blood of pregnant rats, but was found in four out of ten parturient rats.3 After administration of the kininase inhibitor dimercaprol (30 mg/kg i.p.), free kinin was present in the blood of six parturient rats. Dimercaprol caused appearance of free kinin in the blood of only one out of six non-pregnant rats.4 It appears that the plasma kinin system is activated to a small degree during parturition.