Five different extrudates were produced that were composed of ground oats, oat flour and corn groats. These substrates were used in different proportions: extrudates E₁ and E₅ did not contain corn groats but oats products only. The remaining extrudates (E₂, E₃, E₄) contained corn groats and oats substrates in similar proportions. In the extrudates as well as in the mixtures of initial components the content was determined of basic components, i.e. of protein, fat, starch and fiber fractions. Extrudates were also subjected to enzymatic hydrolysis in vitro to determine the protein and starch digestibility coefficients. It was shown that the process of extrusion did not cause significant changes in the content of protein total and starch and it decreased raw fat content. A favourable effect was found of extrusion on a decrease in the amount of insoluble dietary fibre and an increase in soluble fibre fraction. In the studies of apparent digestibility, it was shown that oat substrates subjected to extrusion had better coefficients of protein and starch digestibility; however, increasing the proportion of ground oats in the extrudates decreased digestibility coefficients.