Delays between time of sampling and time of testing are common; therefore, the length of time that blood can be stored at various temperatures was evaluated for effects on cholinesterase activity. Six horses were treated with 16 g of trichlorfon per os, 6 horses were treated with 15 g of dichlorvos per os, and 10 horses were untreated controls. The cholinesterase activity in whole blood from each horse was measured using an adaptation of the Ellman colorimetric method. The blood from each horse was then divided into 3 groups and stored at 5 C (refrigerated), 20 C (room temperature), or 38 C (incubated). Subsequent cholinesterase activities were measured daily and then at weekly intervals. The cholinesterase activities did not significantly increase or were measured daily and then at weekly intervals. The cholinesterase activities did not significantly increase or decrease (P > 0.05) in the blood from the untreated horses until after 1 week for any of the 3 temperature groups. The cholinesterase activities did not significantly increase or decrease (P > 0.05) in the stored blood from the trichlorfon-treated horses for 4 weeks in all 3 temperature groups. The cholinesterase activities significantly increased (P < 0.05) in the stored blood from the dichlorvos-treated horses after 1 week when the blood was refrigerated and by 24 hours when the blood was stored at room temperature or incubated. Therefore, blood from normal or organophosphate-treated horses can be used for cholinesterase evaluation for up to 1 week when stored at 5 C.