Cardiac output, organ blood flow and organ weights were examined in rats assigned at d 0 of lactation to a control (C) group fed ad libitum or an acutely restricted (AR) group fed 50% of the intake of C dams. Dams in each group were assigned to subgroups for measurement of milk yield or cardiac output, blood flow and organ weights. At d 14 of lactation, cardiac output and blood flow were measured with radiolabeled microspheres and milk yield with the tritiated water method. In AR dams cardiac output was 55% of that of C dams, but cardiac output relative to body weight did not differ between groups. Mammary gland blood flow and weight were reduced in AR dams. The weight of the kidneys, gastrointestinal tract and liver of the AR dams was less than that of C dams; however, relative blood flow to these organs did not differ between groups. Milk yield was reduced by 58% in AR dams compared to C dams. We conclude that dietary restriction during lactation negatively affects absolute cardiac output, blood flow to the mammary glands and milk yield, and that the reduced milk yield is associated with the decrease in mammary gland weight and blood flow.