Within about 12 hours after hatching, the smallest of the three owlets (III) weighed 48.7 grams. During the first four weeks of postnatal growth, each owl gained in weight, daily, an average of 33-1/3 grams or an increase of 11.1 per cent. Owl I gained an average of 36.1 grams each day, or a daily increase of 10.7 per cent; owl II, 37.8 grams, or 11.2 per cent; and owl III, 26.1 grams, or 11.4 per cent. From the beginning of the fifth week until the time the young left the nest, the three owls gained on the average only 12.7 grams or approximately 1.6 per cent in weight daily. Individually, the daily mean increases were as follows: I, 9.6 grams or 0.93 per cent; II, 12.7 grams or 1.86 per cent; III, 15.8 grams or 1.97 per cent. Prior to the twenty-sixth to twenty-eighth day of age, there seldom was any loss in weight from day to day, whereas after this period, approximately one weight in four was less than on the previous day. These data support the earlier statement that during the first 3-1/2 or 4 weeks, there is a relatively uniform and rapid increase in weight whereas after this period weight fluctuates. Growth as measured by changes in weight in these young Great Horned Owls parallels growth in some other young birds. For example, nestling Red-tailed Hawks, as reported upon by Fitch, Swenson, and Tillotson (1946:215), increase in weight rapidly for about the first three weeks and then more gradually. Sumner's (1929b) graphs indicate the same pattern of growth in the Barn and Great Horned owls and Red-tailed and Cooper hawks. Pigeons, judging from the growth curves for bodily weight as given by Riddle, Charles, and Cauthen (1932), increase in weight rapidly until somewhere between the twenty-fourth and thirty-second day of postnatal development. However, in the Golden Eagle, the early part of postnatal development is not one of rapid growth, judging from Sumner's diagram (1929a:164), but after the fourth week there is a rapid increase in weight. Graphs that Sumner (1929b) gives for Sparrow Hawks, Long-eared Owls, and Screech Owls, indicate that in these instances also the increase in weight during the first few days of postnatal development was not so rapid as it was after the end of the first week. Stoner (1935) indicates that in the young Barn Swallow, increase in weight was most rapid between the fourth and tenth days, with the young remaining at the nest until the twentieth day. Much the same pattern of weight increase was noted by Stoner (1945) in the Cliff Swallow. Huggins' (1940:228) sigmoid curve for increase in weight in the House Wren indicates that the period of rapid growth in this species does not begin until the second day. Sumner (1934:284) cites other studies which he believes, for altricial birds, indicate three periods of growth, an initial period of rather slow gain, a period of maximum increase in weight, and a final period of fluctuations. As previously indicated, for