The main objectives of this study were to determine the 1,2-dehydropyrrolizidine alkaloid (DHPA) content in Senecio grisebachii Baker (Compositae), to experimentally demonstrate its toxicity in calves and to describe the main clinical and pathological findings of this toxicity. S. grisebachii plants were collected in Paysandú, Uruguay. The concentration and identification of DHPA and associated N-oxides were determined using liquid chromatography-mass spectrometry. Three calves weighing 85–89 kg received doses of 15, 24 or 45 g of dry S. grisebachii per kg of body weight for 6, 10 or 20 days of treatment, respectively. Two animals received no treatment and served as controls. The animals were clinically evaluated, and blood samples were taken to study the serum levels of gamma glutamyltransferase (GGT), aspartate aminotransferase (AST) and alkaline phosphatase (FAS). After death, necropsy was performed and organ samples were taken for histopathological examination. The concentration of DHPA in S. grisebachii was found to be 0.29% (dry weight basis) as free base and 0.08% as N-oxide for a total DHPA concentration of 0.37%. Individual alkaloids identified included seneciophylline, senecionine and retrorsine. The disease was clinically characterized by depression, anorexia, emaciation, colic, dehydration and death in the three animals. Serum concentrations of GGT, AST and FAS were higher than normal beginning on day 7 after start of treatments. Necropsy findings included generalized edema, hemorrhage, ascites and a grayish liver with increased consistency. The main histological lesions were hepatic necrosis, fibrosis, hepatomegalocytosis and bile duct proliferation. The control calves showed no clinical signs of disease.