Duplicate experiments were conducted to determine the effect of incorporation of rapeseed oil into the diet of the chick on liver size, hydroxyproline content of liver protein, lipid content of the liver and the fatty acid composition of neutral liver lipid and liver phospholipid. For comparative purposes the fatty acid composition of carcass lipid was also determined. The diets fed contained high erucic acid rapeseed oil (HER), low erucic acid rapeseed oil (LER), or sunflowerseed oil (SFO) and were formulated by substitution of 20 parts oil isoenergetically for glucose. All chicks were fed the experimental diets from 4 to 28 days of age. When fed ad libitum, it was found that chicks fed diets containing HER consumed fewer calories and grew at a slower rate than LER or SFO fed chicks. Results also showed that growth and caloric consumption of chicks fed diets containing LER was intermediate between and significantly different from that of chicks fed either the HER or SFO containing diets. Chicks fed diets containing HER had significantly heavier livers than chicks fed LER containing diets, which in turn, were found to have significantly heavier livers than chicks fed diets containing SFO. Results of liver lipid determinations showed that chicks fed diets containing HER had a significantly higher percentage of lipid in their livers than did the SFO fed controls. The level of lipid in livers of chicks fed LER was intermediate between but not significantly different from that of chicks fed diets containing SFO and HER. Level of phospholipid in liver lipid was similar irrespective of whether diets contained HER, LER or SFO. No difference was observed in the percentage of hydroxy- proline in liver protein of chicks fed HER, LER and SFO, thus indicating that collagen, the chief constituent of connective tissue, was not increased. Marked differences in fatty acid composition of liver neutral lipids and phospholipids were observed when diets containing HER, LER and SFO were fed. Both the neutral lipids and phospholipids in liver of chicks fed diets containing HER and LER were characterized by having lower levels of stearic, linoleic and arachidonic acids and higher levels of oleic acid than when diets containing SFO were fed. The liver neutral lipids and phospholipids of chicks fed diets containing HER differed from those of chicks fed diets containing LER in that they contained erucic acid and also had higher levels of eicosenoic and lower levels of linoleic and arachidonic acids. Whether changes in fatty acid composition of phospholipids contribute to structural - functional changes in the liver and to the observed increase in liver size when diets containing HER and LER rather than SFO were fed is unknown. Liver neutral lipids of chicks were found to be more saturated than carcass lipid and when HER was fed liver neutral lipids were found to contain slightly but significantly lower levels of erucic and eicosenoic acids, than carcass lipid. Differences in rates of oxidation of palmitic and erucic acid by extrahepatic tissue may contribute to these differences and thus do not necessarily negate the hypothesis that in the chick the liver helps to protect other tissues from excessive exposure to erucic and eicosenoic acids.