The time course of cardiovascular changes in lactation in the rat.

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.