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The adverse effect of blood-sucking dipterous insects (gadflies, gnats, black flies, midges) was determined and checked, as exerted on the working performance of draft horses working in forests; the possibilities of protecting horses with repellents were studied at the same time. The trial was conducted under field conditions in the territory of the Broumov Forest Establishment in the Náchod and Trutnov districts. A 10% water emulsion of the repellent, diethyltoluamide, was used; the horses were treated with the emulsion in a 24-hour interval. Over the seventy working days from the 1st of July to the 30th of September, 1981, the performance of the test horse increased by 153 m3 wood in the skidding operation (i. e. by 58.06%), as compared with the control group. Recalculated per horse/day, this improvement is 2.19 m3 wood and the saving of prime costs amounts to 35.50 Czechoslovak crowns. The coefficient of loss per untreated horse was 9180 crowns. The effectiveness of the costs (per each crown of the capital expended) was calculated to be 309.07 crowns. The results were statistically significant.