SummaryPlasma thromboxane B2(TXB2) the stable inactive metabolite of thromboxane A2(TXA2), was measured daily by specific radioimmunoassay in three groups of animals before and after experimental infection withStrongylus vulgaris. Infection of four ‘parasite naive’ foals produced a typical acute syndrome with intermittent but statistically insignificant rises in TXB2levels. Interpretation of results was complicated by the presence of a non‐septic peritonitis associated with implantation of the foals with electrodes for recording myoelectrical activity. In two foals of similar age, with some natural exposure toS. vulgaris, there was little or no clinical response to infection and increases in TXB2were absent. Baseline levels were also much lower, indicating that the peritonitis may have affected the results obtained in the first group of foals. Severe mesenteric arteritis was confirmed at necropsy in all six foals. A third group of yearling horses, all with natural exposure to the parasite, were generally resistant to infection. One animal developed arteritis with clinical signs of diarrhoea and mild colic, and also showed intermittent increases in TXB2. The mean plasma TXB2level after infection was significantly higher than in the control period, although absolute levels were lower than those recorded in the ‘parasite naive’ foals. Other animals in this group had low TXB2levels and minimal arteritis was found at necropsy. These results indicate that although infection appears to have an effect on plasma TXB2, the changes are inconsistent and not reliable indicators of the presence of verminous arteritis. The results also confirm the difficulty in establishing infection and the variability of the response in animals with previous