1 Inhibition of cholinesterases (ChE) and carboxylesterases (CaE) by paraoxon (Px) was studied in vitro in the serum, liver, lung and muscle of mouse, guinea pig, rabbit and man (serum only). Moreover, the role of Px hydrolyzing enzyme (Pxase) in the detoxification of Px was studied by inhibiting its activity with EDTA. 2 The ChE and CaE activities as well as their sensitivity to Px varied in different tissues and species. The ChEs were more sensitive than CaEs to Px except in the liver. The CaE activity in human and rabbit sera was low and resistant to Px, indicating that it may have a minor importance for the binding of Px. 3 The Px-inhibited ChEs were spontaneously reactivated in the mouse and rabbit sera during 24 h. In mouse, also the CaE activity was recovered. The presence of EDTA in the incubation medium prevented this reactivation indicating that Pxase takes part in the reactivation process. 4 In rabbit, the serum Pxase activity was very high suggesting a good Px detoxifying capacity of the rabbit serum. 5 The results show that amounts and sensitivities of esterases to OPs in rodents may markedly differ from that in man. Possible species-related differences in the affinity of ChEs and CaEs for OPs and the OP hydrolyzing activity should be taken into the consideration, when animal data are extrapolated to man.