In order to evaluate the allelopathic potential of the early developmental stage of rice plants (Oryza sativa L.), a search for allelopathic substances was undertaken in acetone extract of 10-d-old rice seedlings. Two growth-inhibiting substances were found in the neutral fraction of the extract after silica gel column chromatography, and one substance was further purified by thin layer chromatography, C18 Sep Pack cartridge and HPLC. The purified substance inhibited the growth of lettuce (Lactuca sativa L.) at concentrations greater than 0.3 μgmL–1. These two substances may be released into the environment, either as exudates from living tissues or leachates from residues of the plant, and act as allelochemicals to other plants, which should be investigated further in the laboratory and the field.