Acidogenesis of dairy wastewater produced volatile fatty acids, mainly acetate and propionate, plus hydrogen as by-product. Zinc (Zn) inhibited acidogenesis at concentrations over 10 mg l(-1); but at 10 mg l(-1), or less, it enhanced acidogenesis slightly. On the other hand, copper (Cu) inhibited acidogenesis at all tested concentrations ranging 5-400 mg l(-1). Production of acetate was inhibited by both metals at all concentrations; but production of propionate and hydrogen was favored at low concentrations of Zn (up to 80 mg l(-1)) and Cu (up to 40 mg l(-1)). Production of hydrogen corresponded with the degradation of carbohydrate and the production of propionate. Overall, Cu was 1.4-4.3 folds more toxic than Zn, according to the overall production patterns of fatty acids and hydrogen as well as degradation patterns of carbohydrate and protein.