Two experiments were conducted with growing chicks. In Experiment 1 the effects of soybean oil and tallow at two dietary concentrations and penicillin supplementation on the utilization of rye and wheat-based diets were studied. Rye-fed chicks, when compared to wheat-fed chicks, had poorer (P<.01) performance and nutrient retention. Substitution of tallow for soybean oil in the rye-fed chicks decreased (P<.01) feed intake, efficiency of feed conversion, and weight gain by 8, 17, and 23%, respectively, while the corresponding reductions in the wheat-fed birds were only 2, 2, and 3%. Digestibility of dietary lipids in the rye-fed chicks was affected by the type and concentration of lipid, as inclusion of tallow at 3 and 8% decreased lipid digestibility by 18 and 34%, respectively, compared with similarly formulated wheat diets. The corresponding reduction in the digestibility of soybean oil was less (9%) and was independent of the dietary levels of soybean oil. Penicillin supplementation of rye diets improved (P<.01) feed intake (7%), feed conversion efficiency (9%), weight gain (19%), and lipid digestibility (9%) while it had almost no effect in the wheat-fed chicks. In Experiment 2, the substitution of sunflower oil, soybean oil, and coconut fat by tallow in rye diets reduced (P<.01) feed conversion efficiency by 18, 11, and 11%, respectively, while the corresponding changes for the wheat diets were only −1,1, and 4%. These studies demonstrate that rye contains antinutritional factors that interfere with the utilization of lipids, particularly when the diet contains a high concentration of long chain saturated fats.