Vermeersiekte or ‘vomiting disease’ is an economically important disease of ruminants following ingestion of Geigeria species in South Africa. Sheep are more susceptible and poisoning is characterised by stiffness, regurgitation, bloat, paresis and paralysis. Geigeria aspera was collected in the Vrede district (27° 25′ 48″ S; 29° 9′ 36″ E), Free State Province. The plant material was dried, milled and the toxic principles, known as sesquiterpene lactones, were extracted and isolated following chromatographic procedures. Even though geigerin and ivalin were previously isolated, an unknown sesquiterpene lactone, isogeigerin acetate, was also purified. Mouse myoblast (C2C12) and rat embryonic cardiac myocyte (H9c2) cell lines were exposed to different concentrations of geigerin, ivalin, isogeigerin acetate and a commercially available sesquiterpene lactone, parthenolide. An in vitro colorimetric assay 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was used to assess cytotoxicity. The median effective concentrations (EC50) indicated that ivalin and parthenolide were significantly (p<0.05) more toxic than geigerin and isogeigerin acetate. A concentration-dependent cytotoxic response was observed in both cell lines, however, C2C12 cells were more sensitive. A high performance liquid chromatography (HPLC) analysis was used to evaluate the in vitro metabolism of parthenolide, following the addition of a mouse liver microsomal fraction. Results revealed that parthenolide, incubated with the microsomal fraction, undergoes enzymatic transformation to form a metabolite. / Dissertation (MSc (Veterinary Sciences))--University of Pretoria, 2020. / pt2021 / Paraclinical Sciences / MSc (Veterinary Sciences) / Unrestricted