Summary. lnterspecific hybrid ducks, obtained by crossing domestic with Muscovy, are crammed and compared to controls. Cramming causes significant differences in the blood serum GOT, GPT, cholesterol and amylase levels. Significant differences in the major parameters are also found between large-liver and small-liver animals and with high weight gain or low gain.The following correlations are : liver size and GOT, r = + 0.583 ; liver size and GPT, r = + 0.490 ; liver size and cholesterol, r = + 0.324. The correlation between relative gain during force feeding and GOT level is r = + 0.483 ; between that gain and GPT it is r = + 0.421.