Lambs of the species Merino mutton sheep kept on a perforated floor received, from their 38th day of life, either fattening feed for lambs ad libitum or fattening feed for lambs ad libitum plus chopped dried green fodder ad libitum (quota 17% of the intake of dry matter). At intervals of three weeks 4 lambs of each group were slaughtered from their 40th to their 166th day of life. The determination of the physical rumen volume of one half of the animals did not show significant differences between the two feeding regimes. The rumen volume showed an approximately linear increase (B = 0,84) with the increasing age of the animals. Hyper- and parakeratotic changes of the epithelialis of the rumen were caused by both feeding regimes. They were distinctly stronger and earlier developed in animals which only received fattening feed for lambs. The measuring of the rumen villi showed significantly smaller villi in all age groups of those animals which had taken in dried lucerne green fodder as a supplement.