Laboratory and greenhouse studies were conducted to determine the response of four plant species to 4-amino-3,5,6-trichloropicolinic acid (picloram). Germination of soybean (Glycine max (L.) Merr., var. Lee) seeds was inhibited, step-wise, by 10−8 to 10−3 M concentrations of picloram. Safflower (Carthamus tinctorius L.) germination was inhibited at 10−3 M; however, radish (Raphanus sativus L.) and barley (Hordeum vulgare L.) were not affected by any concentration studied (10−6 to 10−3 M). Measurement of several parameters of growth such as fresh and dry weight, shoot height, number and dimensions of leaves, percent necrosis, hypocotyl or epicotyl length, and hypocotyl diameter revealed that picloram was both growth promotive and inhibitory to safflower, radish, and barley. Levels of picloram required for growth promotion or inhibition varied with the species, stage of plant growth, herbicide concentration, the specific tissue, and the parameter being measured. Typical malformations such as twisting of stems, death of growing points, and occurrence of callus growth (depending on the species and herbicide dosage) were observed in picloram-treated seedlings. Studies on water loss by evapotranspiration suggested that picloram caused a possible alteration of stomatal behavior and/or a disturbance of water metabolism in plants under appropriate conditions.