Lactating female rats are known to exhibit a reduced or 'buffered' plasma corticosterone response to stress. The depressed response is a post-partum phenomenon seen only in the lactating rat with pups, since a lactating rat whose pups have been removed for 8 days gives a normal response to ether stress. Thus it appears that lactation may be necessary for a reduced response to stress, and the reappearance of the normal response coincides with the time when the animal resumes its oestrous cycle. Maternal behaviour by itself could be ruled out as a causative factor since a normal adrenal response was seen in virgin rats which had been sensitized to pups.A depressed corticosteroid response to ether stress was obtained after ovariectomy or treatment with prolactin (2 mg/day for 5 days). However, the effect of ovariectomy on plasma corticosterone levels after ether stress could be reversed by the injection of 5 \g=m\g oestradiol benzoate daily for 5 days.The minimum effective dose of dexamethasone phosphate that was necessary to prevent increased plasma corticosterone levels after ether stress in the normal female rat was 400\g=m\g/100g body weight whereas a lactating rat required only 6\m=.\25 \ g=m\ g and a virgin rat treated with prolactin required 25 \g=m\g. It is apparent that prolactin is acting, in part at least, to increase the sensitivity of the negative feedback system.

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to fill the space, and the incision was sutured. The same procedure was followed with animals subjected to sham operation, except that the bulbs were not disturbed following the breaking of the membrane. At the termination of testing, the animals were killed and their brains were macroscopically examined to determine the presence of olfactory bulb tissue.In the initial experiment, the bulbs were removed from ten virgin and ten non-lactating primiparous rats. Bulbectomies were performed on the primiparous rats approximately 1 month after the litters were weaned. A comparable number of control rats were subjected to sham operation. Twenty-four hours later, three 2-day-old young were introduced, and the adults were observed for 15 min. The young were left in the cages overnight and the adults were again observed the next morning. Females were scored as maternal if they retrieved all the young within 15 min and thereafter were seen to crouch over * Present address: The Medical