The effect of chlorpyrifos application was studied in a high-testosterone (testosterone-treated bulls) and a low-testosterone group (corn oil-treated steers). Frequent sampling of blood before and after 2 chlorpyrifos applications was used to monitor plasma testosterone concentrations and blood cholinesterase activities. Bulls had significantly higher testosterone concentrations (P less than 0.01) than did the steers, before and after the 1st and 2nd chlorpyrifos applications. Bulls had higher cholinesterase activities (P less than 0.01) than did steers before the 1st chlorpyrifos application. However, cholinesterase activity decreased more in bulls when compared with that in steers (P less than 0.01) after the 1st and 2nd chlorpyrifos application. Abnormal clinical signs were not observed in the steers, but 2 of 4 bulls had severe clinical signs of organophosphorus insecticide toxicosis after the 2nd application. Seemingly, chlorpyrifos is more toxic for testosterone-treated bulls than for corn oil-treated steers of similar age and weight.