The effect of the perorally ingested pesticide bentazone (195 mg.kg-1) of Czechoslovak origin on the amino acid composition of proteins of bacteria adhering to the ventral and dorsal ruminal wall investigated in six sheep. Proteosynthesis of adherent bacteria was studied by a modified and quantified elution method (elution of bacteria by an isotonic buffered solution at 4 degrees C). By the latter, a concentrate of undamaged bacteria a adhering to the sheep ruminal epithelium could be obtained. The yield of the method was estimated by scanning electrone microscopy and it was 93.3% (Legáth et al., 1990). The high correlation coefficient (r = 0.94, p < 0.01) suggested that in comparison with the control group of sheep, bentazone in the diet did not have a marked effect on the amount of amino acids in hydrolyzates of bacteria adhering to the dorsal and ventral ruminal epithelium. From the analysis of the single amino acids, it however follows that bentazone in the feeding ration caused significant changes in the concentrations of some amino acids in the proteins of bacteria adhering to the epithelium in the ventral and dorsal part of the rumen (p < 0.05) (Tab. I, II). In both topographico-anatomical parts of the rumen phenylalanine levels significantly increased whereas those of alanine and glycine decreased (Fig. 1). Pesticides can be one of the factors that negatively affect the biosynthetic processes in the rumen of ruminants.(ABSTRACT TRUNCATED AT 250 WORDS)