Muscovy duck has a higher ability to develop hepatic steatosis than Pekin duck. This could result from a lower ability of liver to secrete lipids and lipoproteins. To test this hypothesis, we measured the liver apoB-VLDL secretion in Muscovy and Pekin ducks ad libitum -fed by injecting a flooding dose of [1-13C]-valine. In a first experiment, we found that the delay required for the secretion of apoB by the liver was similar for Muscovy and Pekin ducks (25 min, approximatively). In a second study, the liver ability for apoB synthesis and secretion in both species was assessed in vivo 55 min after the injection of the flooding dose of [1-13C]-valine. The fractional apoB-VLDL synthesis rateswas similar for Muscovy and Pekin ducks ( P = 0.449). However, TG concentration measured in plasma VLDL 3 h after the last meal was higher in Pekin ducks than in Muscovy ducks. Pekin duck seemed to limit the accumulation of lipids in liver by increasing the TG content in VLDL withoutincreasing apoB secretion. Consequently, plasma lipids couldparticipate to the higher lipid deposition in adipose and muscle tissues. However, the concentration of TG-VLDL in plasma results from a balance between liver secretion, peripheral tissue uptake and liver uptake. These last mechanisms require further investigation.