The ability of obidoxime to reactivate organophosphate-inhibited cholinesterases was studied in pigs treated with either trichlorfon, dichlorvos or coumaphos. In 6 pigs Cholinesterase activity was measured in the blood samples both before and after in vitro reactivation with obidoxime. Three pigs were treated with obidoxime 6 h after administration of the organophosphates in order to study the possibility of in vivo reactivation. The results show a close correlation between the ability of obidoxime to reactivate the inhibited cholinesterases in vitro and in vivo. However, there was a marked difference in the possibility of reactivation between the 3 organophosphates. Thus no reactivation was possible after treatment with dichlorvos, while reactivation could be achieved for at least 6 h after administration of trichlorfon. After coumaphos treatment reactivation with obidoxime was possible for more than 24 h.