The increased use of urea fertilizer and substituted ureas herbicides, the implication of soil urease in the effectiveness of urea applied as fertilizer, makes necessary to investigate their relationship. All herbicides investigated, fenuron, monuron, diuron, linuron, siduron and neburon are urease inhibitors. The inhibition constant value depends on molecular groups on the urea skeleton. There is a linear relationship between the Hammett sigma values and log Ki for fenuron, monuron and diuron. The presence of a large hydrophobic group and of one or two chlorine—an electron withdrawing group—on the phenyl ring of the herbicides molecule influences the Ki value. The hypothesis is proposed that the enzyme molecule reacts with inhibitors by means of the oxygen atom of the carboxyl group in the substituted ureas.