Two replicates of three multiparous crossbred Large White x Landrace lactating sows were used to develop a technique for the continuous direct measurement of the blood flow through the mammary gland using transit time ultrasound. Four to six days after farrowing, an ultrasonic transit time flow probe was implanted around the right external pudic artery in order to measure the short-term variations of mammary blood flow through this vessel in response to postural change (standing vs lying), meal distribution, hand-milking, and weaning. After surgery, all sows were fed 3.8 kg/d of a lactation diet and housed either at 20 or 28 degrees C. The implantation of the ultrasonic blood flow probe was successful in all six operated sows. Postmortem examination did not indicate the presence of infection, any collateral bypassing the flow probe, or a reduction of artery diameter. The right pudic artery mammary blood flow (PMBF) was measured for 8.5 h over two periods of three days (d 11 to d 13 and d 18 to d 20 of lactation). The PMBF averaged 910 +/- 238 mL/min but was variable within 1 d. Compared with the lying position, PMBF was decreased (- 6%, P < 0.05) when sows were standing. Between 0 to 15 and 16 to 30 min after oxytocin injection (t = 0) and hand-milking, PMBF remained constant (P = 0.05; 801 vs 767 mL/ min) and increased (P = 0.02), respectively, in comparison with the mean calculated over the preceding 30-min period (982 vs 784 mL/min). The PMBF increased (P < 0.05) after meal distribution and reached a peak 65 min later (i.e., 980 mL/min). The PMBF decreased regularly after separation of piglets at weaning; at 8 and 16 h after weaning, PMBF was 60 and 40% of the value recorded before weaning, respectively. Assuming that PMBF drains one-quarter of the whole mammary gland, it can be calculated that blood flow through the entire mammary gland averages 3.6 L/min and that about 470 L of blood are required to produce 1 kg of milk. The proposed methodology constitutes a new technique to measure direct mammary blood flow and its short-term factors of variation.