[en] A tracer technique was used for root studies under field conditions on a chernozemic soil in Romania. ₇₅Se was injected at the stem base and radioassayed for its presence in soil profiles with a gammasond lowered to different depths. Based on the assumption that ₇₅Se is preferably transferred within the root system to active root tissue of injected plants, the root bioactivity was estimated for corn at the knee high stage and just before tasseling, and for sunflower at early maturing, the crops being subjected to different N-fertilization and irigation treatments. The pattern of root bioactivity varied with crop, time and treatment applied. The technique, which is briefly described, seems to be a promising tool for delineation of root response to variation with depth in the soil profile of moisture and nutrient status and as shown in this pilote investigation for delineation of effects of irrigation and N-fertilization. (Authors)