An in vivo study was carried out to elucidate the partitioning of hepatic fatty acids between oxidation and esterification in normal-fed and 24h-fasted rats. The technique of selective labeling of hepatic fatty acids in vivo was used to monitor non-invasively the metabolism of fatty acids in the livers of awake unrestrained rats. The incoporation rates of [^(14)C) label into blood, muscle, ad~ose tissue, liver glycerolipids and exhaled carbon dioxide was measured after injection of labelled lipoprotein and Triton WR 1339 into rats through a jugular cannula in normal-fed and 24h-fasted rats. The incorporation rate of [^(14)C] label into blood, muscle, and adipose tissue was low(2%) at 15 min after injection in both normal-fed and 24h-fasted rats. After 60 min, 29% and 5.8% of the injected [^(14)C) label had been taken up by rite blood, and 26% and 11 % by the liver in normal-fed and 24h-fasted rats, respectively. There were significant differences between normal-fed and 24h-fasted rats(P$lt;0.05). There was a high incorporation rate(93%o) of [³H] label into liver glycerolipids in both normal-fed and 24h-fasted rats. In contrast, the incorporation rate of [³H] label into blood glycerolipids was low(2-3%). There was no significant difference in the partitioning of hepatic fatty acids in vivo between oxidation and esterification by different pore size tilters(0.20㎛, 0.45㎛) in the normal-fed rats. This result shows that the partitioning of hepatic fatty acids between oxidation and esterification differs depending on physiological conditions of the animal and that the lipoprotein solution(LPS)-filter pore size did not have any influence on monitoring the metabolism of hepatic fatty acids in vivo.