The alkaloidal residue obtained after extraction of dried and pulverized Senecio nemorensis ssp. fuchsii contains two pyrrolizidine alkaloids, i.e. fuchsisenecionine and senecionine. In a chronic experiment doses of 8 mg/kg and 40 mg/kg b.w. of the alkaloidal extract were given five times a week by gavage to male and female Sprague-Dawley rats. The study was terminated after 114 weeks. Repeated serum analysis demonstrated a dose-related elevation of serum glutamic oxalacetic transaminase (SGOT), serum glutamic pyruvic transaminase (SGPT) and alkaline phosphatase (AP). Beside its hepatotoxic effect, the treatment proved to be hepatocarcinogenic, showing female rats to be more sensitive than males. Tumors diagnosed were of hepatocellular and angiogenic origin. In age-adjusted analysis the occurrence of these neoplasms revealed a clear dose-related trend (p less than 0.00005). In the in-vitro mutagenicity test system with V 79 Chinese hamster cells, the alkaloidal extract exerted a weak but dose-related mutagenic activity. Under the conditions used the alkaloidal extract of Senecio nemorensis ssp. fuchsii is a genotoxic carcinogen.