The plasma corticosterone levels were measured in nursing female rats which had been removed from their pups for 12 hrs, at 9 a. m. every day till weaning (day 22). The mean value found in this condition (14.6 +/- 2.7 mug/100 ml) does not differ significantly from that found for the cyclic female (13.7 +/- 4.2 mug/100 ml). 2. Suckling for a period of either 3 or 30 min increases the content of plasma corticosterone to the same value ; this value is maximal (+ 22.8 mug/100 ml) 30 min after suckling has started. 3. When mothers have been anaesthetized by a pentobarbital injection 40 min before suckling, plasma corticosterone increase, measured in the same conditions, is lower (+ 12.3 mug/100 ml). 4. The response to psychic (+ 25.5 mug/100 ml) or systemic (+ 46.6 mug/100 ml) stresses in mothers which have been removed from pups for 12 hr is impaired with respect to cyclic females (+ 62.7 mug/100 ml and + 83.8 mug/100 ml respectively). 5. Psychic stress applied immediately after suckling does not increase the plasma corticosterone level ; systemic stress does so slightly (+ 25 mug/100 ml). 6. In lactating rats the response to stress of the hypothalamo-pituitary-adrenal axis seems to be the same as that of intact or castrated males or ovariectomized females ; but differs from the response of cyclic females or of pregnant females when their basal corticosterone level is low.
