Laboratory studies were conducted to investigate the mode of action of a new herbicide, isouron (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-N,N-dimethylurea). These studies included two plant metabolites of isouron, a monomethylurea (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-N-methylurea) and a urea derivative (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-urea). The compounds were examined for their effects on four metabolic processes of enzymatically isolated soybean (Glycine max L.) leaf cells. The processes examined were photosynthesis, protein, ribonucleic acid (RNA) and lipid synthesis. Relative incorporation of radiolabeled precursors was the parameter measured to assess herbicidal effects. Results indicate that isouron and its monomethylurea derivative possess similar herbicidal properties. The urea derivative revealed substantially reduced herbicidal effectiveness. The primary mode of action of these compounds appears to be inhibition of photosynthesis. Experiments to determine an I₅₀ value for isouron and experiments to evaluate the influence of an antioxidant EDU (N-(2-(2-oxo-1-imidazolidinyl)ethyl)-N’-phenylurea) and a singlet oxygen quencher DABCO (1,4-diazobicyclo-(2,2,2)-octane) on the photosynthetic inhibition and chlorophyll breakdown induced by isouron were also conducted. An I₅₀ value of 0.51 uM was calculated for isouron. There was no significant influence of EDU or DABCO on isouron toxicity. Greenhouse experiments were conducted to assess the interactions of isouron and four herbicide antidotes on two varieties of corn (Zea mays L.), Dekalb 'XL72AA' and 'XL67', and wheat (Triticum aestivum L.), 'Tyler' and 'McNair 1003'. Antidotes used were naphthalic anhydride (1,8 naphthalic anhydride), R-25788 (N,N-diallyl-2,2-dichloroacetamide), CGA-43089 (α-((cyanomethoxy)imino)benzene acetonitrile) and CGA-92194 (α-((1,3-dioxolan-2-yl-methoxy)imino)benzene acetonitrile. Statistically significant antagonistic interactions between herbicide and antidote were observed in both wheat and corn. Synergistic interactions were also recorded. The order of antidotal effectiveness on corn was determined to be: NA > CGA-43089 > CGA-92194 > R-25788 and in wheat: NA > CGA-43089 > R-25788 (CGA-92194 severely limited germination). Visual observation of both corn and wheat plants indicated that nonantidoted isouron was very toxic to these crops. / Master of Science