Acknowledgmentof Chemistry, for facilities and for permission to publish the results. Mr. A. S. Sethi for some assistance with the analytical work.Laboratory experiments have shown that the pyrethrins are substantive to cattle skin and hair. \\-hen cattle are passed through a spray race, the concentration of pyrcthrins in the circulation liquor decreases. The adsorption rate constant has been calculated for standard conditions and an equation derived for calculation of the concentration after passage of cattle through the spray race. The use of this equation in veterinary practice is discussed.