Arachidonic acid cyclooxygenase metabolites, thromboxane B2 (TXB2), prostaglandin E2 (PGE2) and 6-keto-prostaglandin F1 (6-keto-PGF1) were measured in horses where anaesthesia was maintained with halothane. Two horses suffering from postanaesthetic myositis were compared with four normal horses. TXB2 and PGE2 levels were higher in mixed venous blood drawn from the myopathic horses. An increase of TXB2 and PGE2 levels appeared when myopathic horses were rolled into dorsal recumbency after a prolonged period of lateral recumbency. One hour after the end of anaesthesia, TXB2 had continued to increase whereas PGE2 decreased. By measurements on blood samples drawn from the brachial vein, we have shown that the rising level of TXB2 in mixed venous blood is mainly due to the increase of TXB2 in blood draining the dependent leg. The origin of the rise in PGE2 is not demonstrated in this study. 6-keto-PGF1 did not change during anaesthesia. An explanation of this imbalance between TXB2 and 6-keto-PGF1 production is considered.