Senecio jacobaea (SJ) was incubated in sheep rumen fluid-buffer mixtures to determine if metabolism and(or) detoxication of pyrrolizidine alkaloids (PA) was occurring. The nontoxic reduction metabolite, 7 beta-hydroxy-l-methylene-8 alpha-pyrrolizidine, was not detected when SJ-rumen fluid incubation extracts were subjected to high performance liquid chromatography and mass spectrographic analysis. Rats were used as assay animals in another experiment to assess the toxicity of SJ incubated in rumen fluid. Incubation treatments were: Rumen fluid (RF) from a sheep not fed SJ (RF-0); RF autoclaved before incubation (RF-0A); RF from sheep fed 50% SJ for 1 wk (RF-1); RF from a sheep fed 50% SJ for 5 wk (RF-5), and RF-5 with 5 microM iodoform in the incubation medium (RF-5I). The SJ treatments were included in rat diets at the 10% level. Dietary treatment and mean rat survival times (days) were: control, no mortality; 10% untreated SJ, 43; RF-0, 53; RF-0A, 55; RF-1, 44; RF-5, 56; RF-5I, 44, There were no significant differences in survival time. This indicates that SJ was not detoxified as a result of incubation in sheep RF in vitro, and suggests that rumen detoxification does not account for resistance of sheep to SJ. The pH and volatile fatty acid concentrations of the incubation mixtures were measured before and after incubation. Acetate/propionate and pH following incubation were respectively: RF-0A, 7.3, 4.3; RF-0, 4.2, 4.4; RF-1, 2.7, 4.5; RF-5, 2.6, 4.5; RF-5I, 2.4, 4.5. These data show that although pretreatment of the rumen fluid donor with dietary SJ and addition of iodoform to the incubation mixture favor reductive rumen metabolism, detoxification of PA from SJ does not occur during in vitro sheep rumen fermentation.