PLA₂s are major components of Viperidae snake venoms. Two myotoxins with PLA₂ structure were isolated from Bothrops asper snake venom: MT-II an inactive Lys 49 PLA₂ and MT-III an active Asp 49 PLA₂. Despite differences in enzymatic activity both evoke inflammatory events. In this study we evaluated the effects in vivo and in vitro of MT-II and MT-III on the expression and catalytic activity of the cyclooxygenases COX-1 and inducible COX-2, and the release of prostanoids. Male Swiss mice were injected i.p. with MT-II or MT-III (1 µg/g) or sterile saline (controls). At selected time intervals after these injections, peritoneal washes were performed for determination of PGE₂ and PGD₂ concentrations by EIA. Expression of the cyclooxygenases was determined by western blot and peroxidase activity was assayed colorimetrically in leukocytes lysates. Results showed that i.p. injection of MT-II and MT-III induced expression of COX-2 but not COX-1 in peritoneal leukocytes from 1 up to 24 h. The expressed protein COX-2 showed a marked peroxidase activity whereas the constitutive COX-1 activity was not modified after the injection of both PLA₂s. Moreover, these toxins significantly increased the levels of PGE₂ and PGD₂ in the peritoneal exudates. In vitro incubation of mice peritoneal resident macrophages (1x10⁶ cells) with MT-II or MT-III (6.5 µg/mL) resulted in increased release of PGE₂ and PGD₂ and expression of COX-2 but not of COX-1 by these cells from 3 up to 6 h of incubation. These data demonstrate the ability of snake venom PLA₂s to induce the expression of active COX-2 protein by peritoneal leukocytes. This effect may be at least in part due to a direct action of both myotoxins on macrophages. Induction of expression of COX-2 may be the major mechanism for the increased production of PGE₂ and PGD₂ induced by both PLA₂s. Since MT-II lacks enzymatic activity, the catalytic activity of some snake PLA₂s may not be relevant for their stimulating effects on cyclooxygenases.