This study was designed to investigate any possible interactions between the mechanisms for the hypercholesterolemic effect of dietary casein and the hypocholesterolemic effect of dietary fish oil(Marinol containing EPA 24.8% and DHA 10.1%) in young chicks. Two feeding trials were conducted using a strain of Single Comb White Leghorn male chicks of about one-week old. The diets in Exp. 1 and 2 were fed ad libitum for 28 and 14 days, respectively. Consumption of casein, compared to that of soy protein, led to a remarkable reduction in feed intake and body weight gain. However, no significant differences were observed between performances of chicks fed the diets containing fish oil or com oil. Consumption of the casein diet caused a significant increase in total plasma cholesterol levels, and that was primarily due to the increased level of lipoprotein cholesterol rather than high density lipoprotein cholesterol. In Exp. 2 where casein was the only protein source, ingestion of the diets with added cholesterol led to a severe hypercholesterolemia mainly because of the increase in the level of plasma low density lipoprotein cholesterol. This response was most remarkable in the chicks fed the com oil diets with added cholesterol. An interesting observation was that chicks fed the fish oil diet appeared to have significantly lower plasma cholesterol levels compared to those of the com oil group(P$lt;0.05). However, fish oil intake was not quite effective in preventing hypercholesterolemia in chicks fed the diet without supplemental cholesterol. These results seem to suggest that the control mechanisms for plasma cholesterol levels by fish oil and casein might not be interrelated.