Fifteen Slovak Merino sheep were included in the experiment. The animals weighing 21-28 kg were divided into three groups per five animals. In a six-week feeding experiment the animals of group I were given 50 mg supermethrin per kg live weight per day while those of group II received 200, and from week four of the experiment 300 mg supermethrin per kg live weight per day. During the experiment changes of aspartate aminotransferase (EC 2.6.1.1), alanine aminotransferase (EC 2.6.1.2), acetylcholine esterase (EC 3.1.1.7), urea und creatinine levels in blood serum were observed. Six weeks after supermethrin treatment the sheep were slaughtered and histochemical evaluation of alkaline phosphatase (EC 3.1.3.2), acid phosphatase (EC 3.1.3.1) and non-specific esterase (EC 3.1.1.1) was carried out in liver, kidney, duodenum, jejunum and ileum. In the course of the experiment changes of the enzymatic activities of aspartate aminotransferase observed in both experimental groups of sheep were similar to those seen in the control group of animals (Tab. I). As compared to the starting values, no significant changes in the activity of alanine aminotransferase were observed in group II of the experiment and in the controls. However, a significantly decreased alanine aminotransferase activity could be seen in the blood serum of sheep of group I (Tab. II). In both experimental groups of animals no significant changes in the acetylcholine esterase could be seen (Tab. III). As compared to the starting values, no significant changes were observed in creatinine levels of the control and the 1st experimental group of sheep (Tab. IV). In the sheep of the 2nd group a temporary significant decrease (p < 0.05) in creatinine levels was seen. The dynamics of urea levels was similar to starting values in all animals throughout the experiment Tab. V). In the control group of animals (Fig. 1) the high density of reaction product of alkaline phosphatase was determined in the microvilli of enterocytes of the small intestine. In the small intestine of the animals of both experimental groups, the activity of this enzyme was shown to be located in the same zone (Fig. 2). In all experimental animals in the parenchyma of the liver and kidney no significant changes could be observed. In both experimental and control animals the high activity of acid phosphatase was demonstrated to be located especially in the cytoplasma of enterocytes. The activity of non-specific esterase was located in the cytoplasma of enterocytes of the small intestine, in the intestinal crypts its activity was slight up to high.(ABSTRACT TRUNCATED AT 400 WORDS)