Pretreatment of dogs with 20 mg/kg of indomethacin blocked some of the hemodynamic effects of endotoxin. In the treated animals intravenous injection of the LD80 dose of endotoxin (Escherichia coli) caused a significantly smaller transient fall in the first phase of shock than in the control group. After the initial brief fall, the arterial blood pressure stayed at normal level for the duration of the experiment in dogs that received indomethacin. This was in contrast to the characteristic decline of arterial pressure in the untreated dogs in the 2nd phase of shock. The results were discussed and compared to those obtained with other anti-inflammatory agents.