Four experiments were conducted with growing chicks to study the effects of feeding rye and wheat diets supplemented with different calcium levels, two types and two levels of fat, and increasing levels of vitamin D3 and to study the interrelationships of these dietary ingredients on chick performance. In Experiment 1, feed conversion efficiency was reduced to a much greater extend (10% vs. 1%) when tallow replaced soybean oil in a rye-based diet containing only .6% calcium compared to a wheat-based diet. When the dietary calcium was increased to 1.1% the replacement of tallow by soybean oil resulted in only a small reduction in feed efficiency for chicks fed either the rye-(3%) or wheat- (1%) based diets. The retention of fat, particularly tallow, was depressed in rye as compared to wheat-fed birds. Fat retention was also influenced by calcium levels. Increasing the dietary calcium from .6 to 1.1% improved tallow (16%) and depressed (7%) soybean oil retention in rye-based diets, whereas in wheat-based diets the retention for both fats was slightly decreased. Experiment 2 showed that the efficiency of utilization of rye diets containing 7.5% tallow was improved by 10% when the level of dietary calcium was increased from .6 to 1.1% while the corresponding improvement in similarly formulated wheat diets was only 1%. The improvement of 2.5% fat, however, was only about 3% for both wheat and rye diets. The results from Experiment 3 showed that in the presence of a large excess of vitamin D3, the depression in the efficiency of utilization of the wheat diet was 13% but was less than 1% for the rye diet. This indicates that rye contains a factor that depresses vitamin D3 absorption. In Experiment 4, when dietary calcium was increased from .80 to 2.00% or when penicillin (200 mg/kg) was added, the improvement in efficiency of feed utilization was greater in chicks fed a rye diet (7%) compared to those fed a wheat diet (1%). The rye diets, however, were always less efficiently utilized than the corresponding wheat diets. Vitamin D supplementation, in contrast, did not affect chick performance. These results show that the performance of chicks fed rye diets is markedly influenced by type and level of dietary fat and the level of dietary calcium.