The inhibitory effect of four trace elements on production of volatile fatty acids (VFA) and lactate was evaluated in cultures of rumen contents containing lead, mercury, cadmium and arsenic at 50, 100 and 250 μg/ml. The production of VFA in cultures containing Pb2+, Hg2+, Cd2+ and As5+ at 50 μg/ml was decreased by 15.9, 40.0, 29.1 and 35.6%, respectively. The effect of trace elements on molar profile of VFA was variable, which suggests a different toxic effect of elements tested on different groups of rumen microorganisms. Increasing concentrations of Pb2+ in rumen cultures decreased molar proportion of acetate and increased that of butyrate, whereas Hg2+ and Cd2 increased molar percentages of acetate and decreased butyrate. Lactate concentration in cultures containing Hg2+ increased in a dose-dependent manner from 3.10 to 9.20 μmol/ml. Effects of other trace elements on lactate production were not consistent.