Selection forestry has a well-established and strong tradition in Slovenia. Animportant issue in Slovenia is as well the overgrowing of agricultural areaswith forests. The aim of our research was to find out, in what manner a pioneer forest, which has developed on agricultural areas, can be transformed into a balanced selection structure through selection silviculture. The research was conducted at the Marinšek estates in Upper Savinja Valley, located between the rivers Savinja and Dreta (region Hom), which belong to theforest management unit Gornji Grad (section 46). On seven permanent research plots in a mature forest, which is in transition from secondary succession at a former agricultural area to a permanent forest area, there were several tasks performed: a full re-measurement of trees, seedlings survey, measurement of light conditions, and drilling of dominant trees. Differences between pioneer and permanent forests were proved with an analysisof regeneration, mixture and structure of forest stands. After a century of development the pioneer forest is more even-structured and richer in species, but less rejuvenated. This is probably because of long-term agricultural exploitation. The method of comparing the diameter distribution to determine the balanced composition of a selection forest is not as sensibleas predicted, whereas as a good method the following were ascertained:the number of trees in the smallest dbh class, number of seedlingsand saplings, mixture of tree species, and the height of growing stock.