Flunixin (FLX) and ketoprofen (KET) are potent nonsteroidal anti-inflammatory drugs (NSAIDs) used to alleviate pain and decrease inflammation. These drugs block access of arachidonic acid to its binding site on the cyclooxygenase enzyme, thus preventing conversion to thromboxane A₂ and subsequent degradation to thromboxane B₂ (TBX). Consequently, plasma TBX may be used to estimate duration of NSAID action. Sixteen adult mallard ducks (Anas platyrhynchos) were randomly assigned to three treatment groups: control (n = 4), FLX 5 mg/kg (n = 6), or KET 5 mg/kg (n = 6). Blood samples were taken 1 hr prior to and just before (0 hr) injection and 0.25, 0.5, 1, 2, 4, 6, 12, 24, 36, and 48 hr after injection. Plasma samples were analyzed for corticosterone and TBX. The feces were tested for the presence of hemoglobin and the ducks were euthanized for complete necropsy at the end of the study. Samples of muscle, kidney, liver, proventriculus, and intestine were taken for histologic analysis. Thromboxane was suppressed significantly in all birds following administration of either FLX or KET for 4 hr and decreased for approximately 12 hr compared with baseline samples (−1 and 0 hr). In the control group, TBX gradually declined over time. None of the ducks showed evidence of gastrointestinal bleeding, but the FLX group had muscle necrosis present at injection sites. FLX and KET likely exert pharmacological effects for at least 12 h. Although degree of TBX inhibition cannot be correlated absolutely with degree of analgesia or anti-inflammatory effects, it is possible that these effects are present during this time. This work suggests that FLX and KET can potentially be used as anti-inflammatory and analgesic agents in waterfowl. However, because of muscle necrosis at the injection site, we do not recommend parenteral use of FLX in ducks.