The energetic cost of a sublethal treatment with chlorpyrifos was estimated by use of direct microcalorimetry to measure metabolic heat in susceptible and resistant strains of the German cockroach Blattella germanica L. Moreover, one of the detoxification enzyme systems known to be involved in detoxification of chlorpyrifos, glutathione-S-transferase, was measured. Individual cockroaches were exposed for 20 min on a glass-surfaces treated with 1.14 μg/cm² of chlorpyrifos. There was no difference in glutathione-S-transferase activity of susceptible or resistant strains after the treatment. The heat production increased in the susceptible strain ≈30 min after exposure and declined again after ≈120 min to the basal level. The energetic cost of the exposure to the insecticide corresponds ≈5 h of normal metabolism. There were no significant differences in heat production after toxic treatment in any of the resistant strains. It was shown that measurement of heat production is a sensitive method to prove toxic reactions after exposure to a low dose of insecticide. The use of microcalorimetry as a promising biomarker technology was shown.