Cenchrus echinatus L., seeded in pots at a depth of four cm, was treated with trifluralin at 0.84 kg/ha, incorporated at depths of 0 to 3 and 5 to 11 cm. Euphorbia heterophylla L, seeded at a depth of one cm, was treated with trifluralin at the same rate, incorporated from 0 to 8 cm, pendimethalin at 1.15 kg/ha and 2.4-D, amine, at 1.44 kg/ha at surface, in preemergence. The plants were observed during the two first weeks and after this material was collected and microscopic observations were made on the affected tissues. Trifluralin enlarged the root tip and the coleoptile diameter, inhibiting their growth in C. echinatus. Swelling of cortical cells was observed. The cells had thinner walls and sometimes were multinucleate. In E. heterophylla there was lateral root inhibition and little swelling of cortical cells. Pendimethalin caused great thickening of the transition zone in E. heterophylla, due to swelling of cortical and epidermal cells, which had thinner walls. Small lateral root inhibition occured. The 2.4-D affected tissue formation, originating cells in the pith center. In some plants an epidermal dislocation was observed.