1. Cardiovascular changes in lactating rats have been traced from the first day post‐partum to the end of the third week of lactation. The pattern of changes showed three phases. 2. Between days 1 and 5 of lactation there were sharp rises in both cardiac output and in the blood flow/g tissue for most organs, but little change in the distribution of the cardiac output. 3. Between days 5 and 15 of lactation cardiac output remained steady. The blood flow to tissues actively involved in the body's response to lactation (mammary glands, liver, gastrointestinal tract) also remained at high steady levels, but the blood flow to other tissues declined due to a redistribution of the cardiac output away from them and towards the growing mammary glands and splanchnic organs. 4. Between days 15 and 22 of lactation there were further rises in both cardiac output and in the blood flow/g tissue for most organs. 5. It is suggested that the increases in organ blood flows that occurred in the first few days after parturition (days 1–5) and at the end of lactation (days 15–22) were largely dependent on increases in cardiac output and may represent the maternal response to rapidly rising demands from the young at these times.

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SUMMARY1. Mammary blood flow was estimated in rats by measuring the cardiac output and the proportion of it received by the mammary glands.2. When the young were removed on the 10th day of lactation the mammary glands began to fill with milk and mammary blood flow fell from 78 ml./min. 100 g tissue to 45 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g in the next 16 hr. These changes were associated with both a fall in cardiac output and a fall in the proportion of the cardiac output taken by the mammary glands.3. When the young were allowed to continue suckling, but milk removal was prevented by sealing the teat ducts with adhesive, more milk collected in the mammary glands within 8 hr and mammary blood flow was unchanged -(74 ml./min. 100 g).4. In rats which had been separated from their young for 24 hr, milk was removed from the engorged glands by allowing the pups to suckle again. Mammary blood flow did not rise immediately following the removal of milk but only after 4 hr of suckling, and was associated largely with an increase in cardiac output.5. Upon resumption of suckling mammary blood flow was the same in emptied glands, and in full glands with the teats sealed.6. When the young were removed from 15-day lactating rats mammary blood flow after 24 hr was directly related to the volume of milk in the glands.7. It is concluded that the accumulation of milk in the mammary gland does not mechanically restrict the flow of blood through the tissue and that, in the rat, mammary blood flow and milk secretion are strongly dependent on a continually applied suckling stimulus.

SUMMARY1. Salt-gland blood flow in the domestic goose has been measured using a combination of Sapirstein's indicator fractionation technique for organ blood flow and Fegler's thermodilution method for cardiac output.2. Nasal salt secretion was induced by giving 0.5 M-NaCl or 0-154 MNaCl i.v. or by giving artificial sea water by stomach tube into the proventriculus.3. During secretion, salt-gland blood flow increased from 82 7 + 21 9 ml./100 g tissue. min to as high as 2179 ml./I00 g. min (mean 1209+ 140).4. The rate of secretion in response to salt loading was very variable and was not correlated with the rate of blood flow.5. From the data obtained, it could be calculated that the median values for the percentage extraction of ions from the arterial plasma were Na 15 %, K 35 %, C1 21 % and water 5-8 %.6. Atropine abolished secretion but not the increase in blood flow produced by salt loading.7. Unilateral complete denervation abolished secretion from and the increase in blood flow through the operated but not the control gland.8. Anaesthesia, induced by pentobarbitone sodium, almost completely blocked secretion and the increase in blood flow in the salt-gland in response to salt loading.9. In geese given 0 5 or 0-154 M-NaCl i.v. a positive, significant correlation was found between the total amount of nasal secretion collected over 30 min and the concentrations of Na and Cl in the nasal fluid. However, when the time course of secretion was followed in any one bird, the rate of secretion was inversely related to the concentrations of Na and Cl.10. Harderian gland blood flow was not affected by salt loading.

SUMMARY1. Factors controlling adaptive hypertrophy, which occurs when marine, or potentially marine, birds drink salt water, have been investigated in geese and ducks using changes in salt-weight weight, RNA and DNA contents as indices of this process.2. Unilateral post-ganglionic denervation in geese prevented the changes