The effect of supermethrin on the overall health with respect to weight gains, diet intake, triad values (body temperature, pulse rate and breathing rate) and potential intoxication signs was investigated in sheep of the Slovak Mertino breed (age of 8 months, males and females) during 6-week feeding of the insecticide supermethrin (Research Institute of Chemical Technology, Bratislava). This insecticide supermethrin contains a cyanide group in its molecule and can be included in the group of type II pyrethroids. It is an analog of cypermethrin and it has a different proportion of cis- and trans-isomers. Supermethrin mixed with molasses feed M was administered daily at a dose of 50 mg/kg (about 1/70 of LD50) to five sheep of experimental group I, at a dose of 200 mg/kg (about 1/15 of LD50) to five sheep of experimental group II, and the dose increased from 200 mg to 300 mg/kg l.w. (about 1/20 of LD50) since the fourth week of trial. The main signs of its toxic action involved depressive effects on weight gains (Fig. 4) Over the whole period of trial, the live weight rose by 5.44 +/- 1.94 kg in control group, by 2.66 +/- 1.48 kg in experimental group I, which equates a significant decrease by 51.10% and only 0.34 +/- 0.95 kg in experimental group II, which equates a decrease in weight gains by up to 93.75% against the control. We do not believe that the growth depression can be related to diet intake. There were no larger differences in feed intake between the experimental groups and the control. The growth depression was caused by incessant diarrhea. The patho-morphological examination did not reveal hyperemia and/or intestinal inflammation, the histological examination did not show any lesions of epithelium in the intestinal mucosa. An increase in supermethrin dose from 200 to 300 mg/kg l.w. resulted in signs coming from the CNS. Hypersensibility manifested by moderate unrest, head and neck shaking after auditory, and especially after touch stimuli was observed. This tremor was increasing to became spontaneous 3-4 days before trial termination. The above findings clearly suggest that supermethrin administration at lower doses has harmful effects primarily on the digestive tract, but at higher doses these effects are more intensive accompanied by the effects on the CNS. No negative effects on pulse rate (Fig. 1), breathing rate (Fig. 2) and internal body temperature (Fig. 3) were recorded.