In anaesthetized rabbits, cardiac output (C.O.) and its distribution to the mammary glands, heart, liver and kidneys have been determined in established lactation (11--13 days), later lactation (26--27 days) and in virgins. During lactation, the volume of circulating blood, C.O., mammary blood flow and mammary weight were significantly greater than in virgins. There were no significant differences in C.O. and % C.O. received by the mammary glands between established and late lactation, and no significant decrease in mammary blood flow in late lactation. The weights of the liver and kidneys were significantly increased in lactation but there were no significant differences in liver, heart (coronary) and kidney blood flow. The rate of growth of the young was positively and significantly correlated with % C.O. received by the mammary glands and mammary weight, but not with C.O. Strong correlation was also observed between the % C.O. received by the mammary glands and mammary weight. There were no significant differences in C.O., mammary % C.O. and mammary blood flow in animals in established lactation 2--3 h and 24 h after suckling (i.e. shortly after and just before suckling). By 48 h after the last suckling mammary blood flow and % C.O., but not C.O., were significantly decreased. Possible factors causing these changes are discussed. The results are discussed in relation to the change in milk composition that occurs in late lactation in this species and to the role and effects of prolactin. It is suggested that events occurring during lactation have different sensitivities to prolactin.