Bioassays were developed and applied to test the role of eicosanoids and pH changes in ecdysis of Oesophagostomum dentatum. Exsheathment (80-100%) was achieved by subjecting third-stage larvae (L3) either to chlorine (hypochlorite assay) for 5 min or by incubating them in HCl followed by addition of NaHCO3 (pH-change assay) with subsequent cultivation at 38.5 degrees C/10% CO2 for 1 week. Addition of the lipoxygenase (LOX) inhibitor diethylcarbamacine (DEC) to the larvae resulted in a reduction of the exsheathment rates which could be restored by the addition of leukotrienes (LT)B4, LTC4, LTD4 and LTE4. Addition of the cyclooxygenase (COX) inhibitor acetylsalicylic acid (ASA) also resulted in decreased exsheathment rates both in the hypochlorite and in the pH-change assays in a dose-dependent manner. However, the primary COX products (prostaglandins) were not able to reverse this effect, in contrast to LTC4. It was concluded that: (1) both tests are suitable for bioassaying the effect of substances on exsheathment, and (2) eicosanoids involved in the control of exsheathment of L3 of O. dentatum are primarily LT.