Different cellular and biochemical cytotoxicity indicators have been assessed to evaluate the damages caused in Vero monkey kidney fibroblasts after 24 h exposure to paraquat (PQ), a widely used bipyridyl herbicide highly toxic through the active oxygen species that it generates by redox cycling. Cell viability, estimated by the relative neutral red uptake (EC50 = 0.5 mM), was more sensitive to PQ than cell proliferation, measured by total protein content (EC50 = 5 mM). Cell growth was more extensively inhibited in the presence of fetal bovine serum than in its absence. PQ exposure was paralleled with higher intracellular specific activities of lactate dehydrogenase and phosphofructokinase, directly assayed in the 96-wells culture plates, whereas those of succinate dehydrogenase raised only 1.35-fold and hexosaminidase was almost unaltered by PQ. The intracellular specific activities of several antioxidative enzymes were also directly determined in the microtiter plates. At the highest PQ concentration used (10 mM) glutathione reductase activity increased 4-fold, while superoxide dismutase and glucose-6-P dehydrogenase activities increased 2- and 1.8-fold compared to untreated control cells. An 1.9-fold raise in glutathione-S-transferase activity was also observed in exposed cells. The results show the action in Vero cells of a complex regulatory defensive network against PQ-induced damages.