A method is described by which non‐volatile contact insecticides may be applied as dusts in a modified Barcroft respirometer with the minimum of disturbance. The technique makes it possible to observe the initial effects of insecticides on insect respiration.This apparatus has been used to study the effects on the respiration of adultTribolium castaneumHbst. of a number of insecticides, including so‐called inert dusts, and other finely divided solids, some of which are used as diluents.At 80% R.H. neither talc, carborundum, Almicide (Al2O3), stockalite, nor decolorizing charcoal caused any appreciable immediate change in the rate of oxygen uptake ofT. castaneum, but after 24 hr. the highly toxic substances, Almicide and charcoal, had caused a marked increase. At 20 % R.H. talc caused an increase in oxygen uptake only after 48 hr., but Almicide was shown to cause an almost immediate slight increase in respiration and a rapid evolution of water vapour. It is suggested that the increase in respiration may result from an increased metabolism to replace water lost through the cuticle.3:5‐Dinitro‐o‐cresol (D.N.C.), p‐nitrophenyl diethyl thionphosphate (E 605), γ‐isomer of benzene hexachloride (γ‐B.H.C), D.D.T., pyrethrins, toxaphene, chlordane, hexaethyl tetraphosphate all caused an increase in oxygen uptake before killingT. castaneumbut lauryl thiocyanate did not. Toxaphene and chlordane required a much longer time to cause an increase in respiration than did any of the other subtances tested.D.N.C. in sublethal concentrations caused an increase in the rate of respiration ofT. castaneum.Concentrations of D.N.C. giving 100% kill caused a rapid increase in oxygen uptake of which the magnitude and duration were independent of concentration.E 605 caused an increase in respiration after a time delay which was dependent upon the concentration used. The magnitude and duration of the increase in the rate of oxygen uptake were not greatly affected by the concentration of E 605.The effects of the concentration of y‐B.H.C. and the pyrethrins are complex and require further study. Both substances can affectT. castaneumwithout affecting respiration. Knock‐down can occur without affecting