The distribution of blood flow in the udder of the sheep and changes brought about by cold exposure and lactation.

1. Microspheres have been injected into an external iliac artery of conscious female sheep in order to measure the distribution of blood flow in half the udder. Values were obtained from five non-lactating ewes in a neutral environment and in a cold environment. Values were also obtained from five lactating ewes in a neutral environment 27-36 days after parturition. 2. In non-lactating sheep in a neutral environment, blood flow to half the udder averaged 11.7 ml . min-1 of which 43% was distributed to lobulo-alveolar tissue, 10% to skin covering the udder, 2% to the teat, 20% to inguinal adipose tissue and 25% to lymph tissue. 3. Cold exposure significantly reduced udder skin and teat blood flow, by approximately the same amount as it reduced leg skin blood flow, but significantly increased blood flow to adipose tissue in the udder, so total blood flow to the udder was not affected. 4. In lactating sheep, blood flow to half the udder was significantly higher, averaging 357 ml . min-1. Flow to the teat was slightly greater than in non-lactating sheep in a neutral environment, but flow to lobulo-alveolar tissue was much greater and accounted for 98% of total blood flow in lactating sheep.