The heritable variation of body weight in two lines of chickens divergently selected for high and low juvenile body weight, as determined from parental line, F₁, F₂ and backcross generations, was principally additive. Feed intake during the first few weeks post hatching was influenced by potence in the direction of the high-weight line, while no comparable pattern was noted for early water intake. Heterosis was observed for feed efficiency which was associated with an equivalent negative heterosis for percentage carcass fat. Feed intake of chicks from the high-weight line and reciprocal crosses were restricted to that of chicks from the low-weight line. This restriction resulted in a greater depression in body weight of the high-weight line than of cross chicks. In contrast, a comparable restriction of water consumption resulted in a similar (10%) depression of body weight among both high-weight line and cross chicks. The effect of feed and water restriction on feed:water ratios was dependent upon mating type. The amount of feed introduced into the crop of chicks from the high-weight line via overfeeding was similar to their ad libitum intake. Chicks from the low-weight line could be overfed at earlier ages and to a greater degree than either high-weight or cross chicks. The degree of overfeeding was associated with the relative size of certain gastrointestinal components. Differences among populations were reflected in concomitant changes in body weight and carcass fat. Comparisons among the parental line and F₁ progeny provided evidence for nonadditive genetic variation for hedonic sensitivity towards quinine sulfate and dextrose, though in opposite directions. Additive genetic variation appeared to influence the preference ratios for both stimuli at super-threshold concentrations. These data suggest the absence of a simple relationship among growth, feed intake control and body composition. It is hypothesized that in addition to their commonality, different genetic systems exist for the expression of high and low body weight as well as their various correlates. The implications of these systems are discussed within the context of feed intake control and future genetic changes in growth. / Ph. D.