The intake of mammary secretion from delivery to day 5 post-partum was determined by test-weighing nine infants using an integrating electronic balance. The mean yield of colostrum for the first 24 h after birth was 37.1 (range 7.0-122.5) g and was 408 (range 98.3-775) and 705.4 (range 452.5-876) g/24 h on days 3 and 5 post-partum respectively. The milk yield of mothers on either day 14 or 28 post-partum was determined by test-weighing the mother. The mean milk yield was 1.156 (SD 0.167) kg/24 h. A significant correlation (P less than 0.001; r 0.85, n 42) was found between milk yield measured by test-weighing the infant and milk yield measured by test-weighing the mother, confirming that it is possible to obtain a similar estimate of milk consumed using either of the two methods of test-weighing. There was a significant positive correlation (P less than 0.001) between lactose concentration and milk yield for the first 5 d post-partum (r 0.76, n 22); a significant correlation (P less than 0.001) between protein concentration and milk yield (r-0.74, n 22) and no significant correlation between fat concentration and milk yield for the period studied. The calculated energy intake of infants during the first 24 h after birth was only 0.12 (range 0.02-0.29) mJ. This increased to 1.44 (range 0.83-2.18) and 2.99 (range 2.49-4.06) mJ/24 h by days 3 and 14-28 post-partum respectively.