Rice (Oryza sativa L., var. Bluebonnet) bioassay was utilized to evaluate activity of several herbicide combinations at concentrations of each individual compound causing a 50% reduction in root length. Synergistic combinations were trichloroacetic acid (TCA) plus 1,1-dimethyl-3-phenylurea (fenuron); TCA plus 3-(p-chlorophenyl)-1,1-dimethylurea (monuron); TCA plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron); TCA plus 3-amino-s-triazole (amitrole); TCA plus 2,2-dichloropropionic acid (dalapon); and dalapon plus monuron. Additive results were obtained from combinations of TCA plus 1,2-dihydro-3,6-pyridazinedione (MH); dalapon plus fenuron; and dalapon plus diuron. MH and (2,4-dichlorophenoxy) acetic acid (2,4-D) exhibited antagonistic responses when combined with fenuron, monuron, diuron, dalapon, amitrole, and with each other. Antagonistic responses were obtained from combination of 2,4-D and TCA.