The organophosphorus insecticide tetrachlorvinphos (TCVP) is commonly used as a feed-through larvicide in many livestock species, including cattle and horses. Cholinesterase (ChE) activity in blood (generally plasma or whole blood) is often employed to assess organophosphorus insecticide intoxication in animals as well as humans. In many species, including horse and man, plasma contains predominantly butyrylcholinesterase whereas red blood cells in all species express exclusively acetylcholinesterase. To evalulate the comparative interaction of TCVP with blood ChEs in different species, we compared the in vitro sensitivity of ChE activity in plasma and erythrocytes from horse, cow, and rat. Horse plasma ChE was most sensitive (IC(50), 30 minutes, 30 degrees C = 97 nM), whereas horse erythrocyte ChE activity was least sensitive (IC(50) > 1 mM). In contrast, cow plasma ChE showed lower sensitivity (IC(50) = 784 microM) to inhibition by TCVP than erythrocyte ChE (IC(50) = 216 microM). Rat plasma and erythrocyte ChE activities had relatively similar sensitivity to TCVP (IC(50) = 54 microM and 78 microM, respectively). The results suggest that plasma and erythrocyte ChE from horse, cow, and rat show marked species- and blood fraction-dependent differences in sensitivity to TCVP. Knowledge of such differences in sensitivity of blood ChE activities to TCVP may be important in the clinical interpretation of intoxication with this pesticide across species.