Intravenous mineral acid infusions into fetal sheep stimulate increases in plasma adrenocorticotropic hormone (ACTH) and cortisol concentrations that correlate to the induced changes in arterial pH (pHa). We have recently demonstrated that ACTH and cortisol responses to mineral acid infusion in adult sheep are mediated by thromboxane A2 (TxA2). We designed the present experiments to test the hypothesis that fetal ACTH and cortisol responses are also mediated by TxA2. We infused chronically instrumented fetal sheep with 1 N HCl (0.5 ml/min i.v.) for 60 min, with or without pretreatment with the cyclooxygenase inhibitor flunixin-N-methylglucamine. HCl infusion significantly decreased pHa and significantly increased the arterial partial pressure of O2 (PaO2) and CO2 (PaCO2). Flunixin pretreatment significantly decreased fetal plasma thromboxane B2 (TxB2) concentrations but did not significantly alter the blood gas and pH response to HCl. TxB2 is a stable metabolite of TxA2 and was measured as an index of TxA2 generation. HCl increased fetal heart rate only in the flunixin group. Plasma ACTH and cortisol concentrations were increased significantly in both groups; flunixin did not significantly alter the responses. HCl infusion did not significantly alter plasma TxB2 concentrations. We conclude that the fetal ACTH and cortisol responses to HCl infusion are not mediated by TxA2 or other prostanoids whose synthesis depends on cyclooxygenase activity.