SummaryThe effects of the herbicide isouron and of its plant degradation products designated as metabolite l {N‐[5‐(1,1‐dimethylethyl)‐3‐isoxazolyl]‐N‐methylurea} and metabolite 2 {N‐[5‐(1,1‐dimethylethyl)‐3‐isoxazolyl]‐urea} on the metabolism of enzymatically isolated leaf cells of soybean [Glycine max(L.) Merr., cv. Essex] were compared under laboratory conditions. Photosynthesis, protein synthesis, ribonucleic acid synthesis, and lipid synthesis were assayed by the incorporation of NaH14CO3, [14C]‐leucine, [14C]‐uracil, and [14C]‐acetate, respectively, into the isolated cells. Time‐course and concentration studies included incubation periods of 30, 60, and 120 min and concentrations of 0.1, 1, 10 and 100 μM of the three herbicides. The urea derivative of isouron (metabolite 2) was the least active of the three compounds. The activity of the mono‐methylated derivative of isouron (metabolite 1) was comparable to that of isouron and the sensitivity of the four processes to both chemicals decreased in the order: photosynthesis>ribonucleic acid synthesis>lipid synthesis>protein synthesis. The concentration of isouron that caused a 50% inhibition of photosynthesis of the isolated soybean leaf cells was calculated at 0.51 μM. The effects of isouron and metabolite 1 on photosynthesis, lipid and RNA synthesis appeared to be independent of incubation lime as maximal inhibition occurred within 30 min. Inhibition of protein synthesis by both chemicals was time‐dependent, increasing in magnitude with c