Polymorphonuclear leukocytes (PMNLs) were isolated from an inflammatory exudate induced in the intercarpal joints of horses by an administration of carrageenin. Their superoxide production at rest and following stimulation with either serum‐treated zymosan (STZ) or phorbol myristate acetate (PMA) was measured by cytochrome‐c reduction. Stimulation of the cells increased the cytochrome‐c reduction 10‐15 times that of resting cells. The maxima were 20 nmol of reduced cytochrome‐c per 106cells per ml at 120 min (STZ) and 35 nmol of reduced cytochrome‐c per 106cells per ml at 60 min (PMA). The maximum inhibition of the cytochrome‐c reduction by superoxide dismutase (Paloseinr̀) was 83.6% (STZ stimulation) and 72.1% (PMA stimulation). The non‐steroidal anti‐inflammatory drugs, phenylbutazone, salicylic acid, aspirin, sodium salicylate in addition to D‐penicillamine and dimethylsulfoxide caused dose‐dependent inhibition of the cytochrome‐c reduction when the cells were stimulated by PMA. The maximum inhibitions were 64% and 36% for aspirin (10‐2m), 32% and 17% for phenylbutazone (10‐3m), 15% and 31% for dimethylsulfoxide (6.4 times 10‐1M), 32% and 19% for salicylic acid (10‐2M), 0% and 17% for sodium salicylate (10‐2M) and 2.2% and 2.5% for D‐penicillamine (10‐4M) when the cells were stimulated by STZ and PMA, respectively. The results indicate that equine PMNLsmigrating from the vascular bed into the joint are a potential source of superoxide and that superoxide dismutase at achievable synovial fluid concentrations following intra‐articularly administered Paloseinr̀, and NSAIDsat concentrations above those achieved in plasma from therapeutic dose rates, may protect tissue from superoxide produced by eq